Physical and Geometric Optics

OPTOMETRY · SEMESTER 2 Nature of light Physical and Geometric Optics START READING NOTES Contents of This Topic Nature of light THE NATURE OF LIGHT LIGHT WAVES VISIBLE LIGHT Wavelength: Long Short Meets Microwave THE SUN’S RAYS THE RAY APPROACH A DIVERGING PENCIL Air 1.001 PHASE DIFFERENCE Nature of light NATURE OF LIGHT CHAPTER CONTENTS THE NATURE OF LIGHT……………………………………………………………………………………………………………………………….. 1 LIGHT WAVES……………………………………………………………………………………………………………………………………………… 3 VISIBLE LIGHT …………………………………………………………………………………………………………………………………………….. 5 THE RAY APPROACH ………………………………………………………………………………………………………………………………….. 7 REFRACTIVE INDEX ……………………………………………………………………………………………………………………………………. 9 PHASE DIFFERENCE…………………………………………………………………………………………………………………………………… 9 THE NATURE OF LIGHT Until the beginning of the 19th century, light was considered to be a stream of particles, emitted by a light source, which stimulated the sense of sight on entering the eye. The chief architect of the particle theory of light was Newton. With this theory, he provided simple explanations of some known experimental facts concerning the nature of light, such as the laws of reflection and refraction. According to Newton, the particles emitted by a source travelled in a straight line until the boundary of a new medium was encountered. He derived a relationship that predicted that the speed of light in a medium such as water would be greater than its speed in air. This is exactly the opposite of the result predicted by the wave theory. Foucault later found a value for the velocity of light in a medium that showed that Newton’s prediction was incorrect. This played a large part in discrediting the socalled corpuscular theory of light.Nature of Light Most scientists initially accepted Newton’s particle theory of light. However, during Newton’s lifetime, another theory was proposed. In 1678, a Dutch physicist and astronomer, Christian Huygens (1629-1695), showed that a wave theory of light could also explain the laws of reflection and refraction. In addition, his theory could account for the fact that light entering a medium from air at an angle would be bent. The wave theory did not receive immediate acceptance for several reasons. All the waves known at the time (e.g. sound and water waves) travelled through some sort of medium; but light from the Sun could travel to Earth through empty space. Furthermore, it was argued that if light were some form of wave, it would bend around obstacles; hence, we should be able to see around corners. It is now known that light does indeed bend around the edges of objects. This phenomenon, known as diffraction, is not easy to observe because light waves have such short wavelengths. Even though experimental evidence for the diffraction of light had been discovered by Francesco Grimaldi (1618- 1663) around 1660, for more than a century most scientists rejected the wave theory and adhered to Newton’s particle theory. This was partly due to Newton’s great reputation as a scientist. The first clear demonstration of the wave nature of light was provided by 1801 by Thomas Young (1773-1829), who showed that under appropriate conditions, light exhibits interference behaviour. That is, at certain points in the vicinity of two sources, light waves can combine. They can even cancel each other by destructive interference. Such behaviour could not be explained by a particle theory. A few years later, and building on Young’s experimental work, Augustin Fresnel (1788-1827) definitely signalled the end of 18th century physics and the birth of modern optics. He extended the wave theory of light to a large class of optical phenomena and developed the theoretical framework that became the foundation of modern optics. The most important development concerning the theory of light was the work of James Clark Maxwell, who in 1865 predicted that light was a form of high-frequency electromagnetic wave. His theory predicted that these waves should have a speed of 3 x 108 m/s. This value is in agreement with the experimentally measured speed. Light is transmitted in the form of transverse waves. In the diagram below we see that the electric and magnetic vectors associated with an electromagnetic wave are at right angles to each other and also to the direction of wave propagation. Figure 1.1: Schematic diagram of an electromagnetic wave propagating in the x direction. The electric field vector E vibrates in the xy plane, and the magnetic field vector B vibrates in the xz plane Although the classical theory of electricity and magnetism explained most known properties of light, some subsequent experiments could not be explained by the assumption that light was a wave. The most striking of these was the photoelectric effect, discovered by Hertz. Hertz found that clean metal surfaces emit charges when exposed to ultraviolet light. In 1905, Einstein published a paper that formulated the theory of light quanta and accounted for the photoelectric effect. He reached the conclusion that light is composed of corpuscles, or discontinuous quanta of energy. Furthermore, he asserted that light interacting with matter also consists of quanta, and he brilliantly worked out the implications of the photoelectric process. Nature of Light More specifically, Einstein showed that the energy of a photon is proportional to the frequency of the electromagnetic wave: E = h·f where h= 6.63 x 10-34 Js is Planck’s constant. This theory contains features of both the wave and particle theories of light. In view of these developments, light must be regarded as having a dual nature. That is, to best describe light’s behaviour, we need in some cases to consider light to be acting as a wave and in others to be acting as a particle. For example, classical electromagnetic wave theory provides adequate explanations of light propagation and of the effects of interference, whereas the photoelectric effect is best explained by assuming that light is a particle. The nature we consider light to have in a particular situation will depend on which is the most appropriate model for those circumstances.. . In this course, we consider Physical Optics which involves phenomena which can only be explained by reference to the nature of light, as well as phenomena which do not depend on the nature of light, but only on its path. This

OPTOMETRY · SEMESTER 2 Vergence Physical and Geometric Optics START READING NOTES Contents of This Topic Vergence DEFINITION SIGN CONVENTION AND UNITS CALCULATING VERGENCE CURVATURE OF WAVEFRONTS EFFECTIVITY APERTURE, RADIUS OF CURVATURE AND SAGITTA LENS CLOCK I Vergence VERGENCE CHAPTER CONTENTS DEFINITION…………………………………………………………………………………………………………………………………………………. 1 SIGN CONVENTION AND UNITS…………………………………………………………………………………………………………………… 2 CALCULATING VERGENCE………………………………………………………………………………………………………………………….. 3 CURVATURE OF WAVEFRONTS ………………………………………………………………………………………………………………….. 4 EFFECTIVITY ………………………………………………………………………………………………………………………………………………. 5 APERTURE, RADIUS OF CURVATURE AND SAGITTA …………………………………………………………………………………… 7 LENS CLOCK I……………………………………………………………………………………………………………………………………………… 8 DEFINITION Much of the work in geometrical optics is concerned with the convergence and divergence of pencils of light rays. In fact, the main purpose of spectacle lenses is to alter the extent of convergence or divergence of light rays before they enter the eye. The convergence or divergence of a pencil of light rays may be expressed by the general term vergence. The vergence at a particular point in a pencil of rays travelling in air is the reciprocal of the distance from the point to the source or the focus. Clearly, we will be dealing with a source in the case of a diverging pencil and a focus in the case of a converging pencil. From the definition, it follows that the closer the point in question is to the source (or focus), the larger the vergence, and vice versa.Vergence The definition of vergence stated above does not distinguish between those points which are situated in converging pencils of rays and those points which are situated in diverging pencils. In order to be able to distinguish between diverging and converging pencils, when studying the effects of optical components such as lenses, a sign convention must be employed. A sign convention is a set of definite rules such that the value of any distance measured on an optical diagram may be given a positive or a negative sign, corresponding to convergence and divergence respectively. SIGN CONVENTION AND UNITS The sign convention for vergence is as follows. 1. In optical diagrams, light is assumed to travel from the left to the right in a positive direction. If the distance from the point in question to the source or focus is measured in the same direction as that in which the rays of light are directed, the numerical value of the distance is given a positive sign (+). 2. If the distance from the point in question to the source or focus is measured in the opposite direction to that in which the light is travelling, the magnitude of the distance is given a negative sign (−). Refer to the below figures. The distance from the point D to the source is measured in the opposite direction to the direction in which the light is travelling and we must apply a minus sign to the distance d. Hence, the vergence at D (= 1/d) will be a negative value. Figure 2.1: Diverging pencil of rays. The distance d is measured from the point to the source, hence from right to left leading to a negative value. However, the distance from the point C to the focus is measured in the same direction as the direction in which the light is traveling and the value of the distance cis assigned a plus sign. Therefore, the vergence at C (= 1/c) will be a positive value. Figure 2.2: The distance c is measured from the point to the focus, from left to right, leading to a positive value.Vergence These rules result in the important conclusion that at any point in a converging pencil of rays the value of the vergence of the light is positive, and at any point in a diverging pencil of rays the value of the vergence of the light is negative. It also follows that at any point in a parallel pencil of rays, for which the source or the focus may be considered to be at infinity, the vergence will be derived as follows: 1 vergence = = 0 ±∞ That is, at any point in a parallel pencil of rays, the value of the vergence of the light is zero. If the distance from a point in a pencil to the source or focus is expressed in metres, then the value of the vergence at that point is expressed in dioptres. We can define one dioptre, symbol 1 D, as being the magnitude of the vergence in a pencil of rays in air, at a point one metre from the source or the focus. CALCULATING VERGENCE Figure 2.3: a) Diverging pencil of rays b) Converging pencil of rays Refer to the previous diagrams of which (a) shows a diverging pencil and (b) shows a converging pencil of rays. The point from which the diverging pencil of rays actually originates will now be referred to as an object. In optics, the distance from a point such as A to an object is represented by the symbol l. Thus the distance from A to the object = l where l is measured in metres. The vergence at A is 1/ l and this is given the symbol L (dioptres). 1 L(dioptres) = l (metres) The point through which all the converging rays pass will be referred to as an image. We will represent the distance from a point such as B to an image by l as well. Care must be taken to apply the sign convention in order to find the correct sign for the vergence. The vergence at B will then also be given by the above equation. For any optical medium with an index of refraction n, the vergence of the light travelling through the medium is given l n L =Vergence It is worth emphasizing that the dioptre unit is really the reciprocal metre (m-1 ). The dioptre is merely a unit of convenience which is easier to say than ‘reciprocal metre’, but when considering units in some equations it will be necessary to think in terms of reciprocal metres. The name dioptre was chosen from dioptrics, the name given to the branch

OPTOMETRY · SEMESTER 2 Refraction Physical and Geometric Optics START READING NOTES Contents of This Topic Refraction WHAT IS REFRACTION? REFRACTING MATERIALS THE REFRACTIVE INDEX AIR OTHER MEDIUM THE REFRACTIVE INDEX VARIES WITH WAVELENGTH REAL AND APPARENT DEPTH TOTAL INTERNAL REFLECTION USES OF TOTAL INTERNAL REFLECTION WHITE LIGHT IS POLYCHROMATIC DISPERSION THE FOLLOWING DERIVATION IS NOT FOR EXAMINATION PURPOSES. COLOUR SELECTIVE TRANSMISSION PRIMARY, SECONDARY & COMPLEMENTARY COLOURS Red 700 – 635 PIGMENTS, PAINTING AND PRINTING OPTICAL PATH LENGTH REFRACTION AT CURVED SURFACES = ̶ 5.88 D = ̶ 6.46 D Refraction REFRACTION CHAPTER CONTENTS REFRACTING MATERIALS …………………………………………………………………………………………………………………………… 0 WHAT IS REFRACTION?………………………………………………………………………………………………………………………………. 0 THE REFRACTIVE INDEX …………………………………………………………………………………………………………………………….. 1 THE REFRACTIVE INDEX VARIES WITH WAVELENGTH ……………………………………………………………………………….. 2 REFRACTION AT PLANE SURFACES……………………………………………………………………………………………………………. 2 REAL AND APPARENT DEPTH……………………………………………………………………………………………………………………… 3 TOTAL INTERNAL REFLECTION…………………………………………………………………………………………………………………… 4 USES OF TOTAL INTERNAL REFLECTION……………………………………………………………………………………………………. 5 WHITE LIGHT IS POLYCHROMATIC ……………………………………………………………………………………………………………… 6 DISPERSION ……………………………………………………………………………………………………………………………………………….. 8 COLOUR…………………………………………………………………………………………………………………………………………………….12 OPTICAL PATH LENGTH……………………………………………………………………………………………………………………………..15 REFRACTION AT CURVED SURFACES ……………………………………………………………………………………………………….15 LENS CLOCK II……………………………………………………………………………………………………………………………………………16Refraction WHAT IS REFRACTION? When a ray of light in air is incident on the surface of a transparent medium such as glass, some of it is reflected, whilst the remainder is transmitted. The larger the angle of incidence is, the larger the amount of light that is reflected. No medium is perfectly transparent and some absorption of light always occurs, the energy being converted to heat. This latter effect will be ignored. The direction of the ray inside the medium is different to that of the incident ray unless the incident ray is normal (perpendicular) to the surface. The transmitted ray is bent as it crosses the surface between one medium and another. This change of direction of the ray at the surface is called refraction. Figure 3.1: Refraction at the air/water and water/air interfaces The preceding diagrams show several important aspects of refraction. When a light beam goes from air into water along the normal to the surface between them, it simply continues along the same path. When it enters the water (an optically more dense medium) at any other angle, it is bent towards the normal. The paths are reversible; thus a light beam emerging from the water (into the optically less dense air) is bent away from the normal as it enters the air. This effect gives rise to the phenomenon of the apparent depths of objects. REFRACTING MATERIALS Common refracting materials are glass, (in particular high-grade optical quality glass), quartz, and a variety of plastics. These have been chosen not only for their effect on the direction of light rays passing through them, but also for their transparency, homogeneity, and their resistance to atmospheric corrosion. Optical glass is commonly used in the manufacture of prisms and lenses. Two main types of optical glass are available: these are crown and flint. The former is a compound of silica (sand, SiO2) and salts of sodium and potassium. In addition, small quantities of other materials such as barium and zinc oxides may be present. Flint glasses, in addition to the constituents above, contain oxides of lead and are denser than crown glasses. Certain plastics materials are now increasingly being used for ophthalmic lenses. In this category is the thermosetting material allyl diglycol carbonate, commonly known as CR-39 (the CR standing for Columbia Resin). These plastics start as linear polymer chains that get cross-linked permanently during molding. Therefore they cannot be remolded. Polyethylene, polystyrene, polyvinyl chloride and polytetrafluoroethylene (PTFE) are examples of thermoplastic materials. These plastics do not undergo any chemical change during the molding process and can therefore be remolded several times without changing their properties. AIR WATER AIR WATERRefraction THE REFRACTIVE INDEX Refraction occurs because light travels at different speeds in different media. We have previously noted that the speed of light in vacuum, c, equals 3 x 108 m/s, which is the maximum speed at which light travels. (We will assume that the light travels with the same speed in air.) In a material medium, the speed of the light, v, is less. The ratio of these speeds is the refractive index (n) of the medium. c n = v Since v is never greater than c, the index of refraction (which is a dimensionless number) is never less than 1. The index of refraction is sometimes called a measure of the optical density of the material. Materials with larger indices of refraction are said to be optically denser. The previous statements about the direction in which light rays are bent in passing into a different medium can now be restated in terms of optical density. In order to account for the light slowing down as it does, consider the accompanying figure which represents a beam of light entering a piece of glass from the left. Once inside the glass, the light may encounter an electron bound to an atom, indicated as point A in the figure. Let us assume that light is absorbed by the atom, which causes the electron to oscillate. The oscillating electron then acts as an antenna and radiates the beam of light toward an atom at point B, where the light is again absorbed by an atom at that point. (We need not consider the details of these absorptions and emissions.) For now, it is sufficient to think of the process as one in which the light passes from one atom to another through the glass. (The situation is somewhat analogous to a relay race in which a baton is passed between runners on the same team.) Although light travels from one atom to another with a speed of 3 x 108 m/s, the processes of absorption and emission of light by the atoms take time. Enough time is required, in fact, to lower the speed of the light in the medium. Once the light emerges into the air again, the absorptions and the emissions cease and its speed returns to the original value. Figure 3.2: A beam of light entering a piece of glass The frequency of a light wave is determined by its

OPTOMETRY · SEMESTER 2 Thin lenses 1 Physical and Geometric Optics START READING NOTES Contents of This Topic Thin lenses 1 TERMINOLOGY OF THIN LENSES FOCAL LENGTHS F2Thin Lenses I F1Thin Lenses I IMAGE FORMATION BY GRAPHICAL CONSTRUCTION DIVERGING LENS LINEAR MAGNIFICATION Object Size (O) CONJUGATE POINTS Thin lenses 1 THIN LENSES I CHAPTER CONTENTS TERMINOLOGY OF THIN LENSES………………………………………………………………………………………………………………… 1 FOCAL LENGTHS ………………………………………………………………………………………………………………………………………… 0 REAL AND VIRTUAL …………………………………………………………………………………………………………………………………….. 2 IMAGE FORMATION BY GRAPHICAL CONSTRUCTION …………………………………………………………………………………. 2 LINEAR MAGNIFICATION……………………………………………………………………………………………………………………………… 5 CONJUGATE POINTS…………………………………………………………………………………………………………………………………… 5 TERMINOLOGY OF THIN LENSES A thin lens is one whose thickness is very small in comparison to its focal length1 . A typical thin lens consists of a piece of glass or plastic, ground so that each of its two refracting surfaces is a portion of either a sphere or a plane. A lens is used to direct or control rays of light. The refraction of light at the surface of a lens depends on its shape, its index of refraction, and the nature of the medium surrounding it (usually air), in accordance with Snell’s Law. Lenses are commonly used to form images by refraction in spectacles and in optical instruments such as cameras, telescopes and microscopes. 1 Although any contact lens must be considered a thick lensThin Lenses I Lenses can be placed in two groups. Those in the one group are thicker at the centre than at the rim. They are known as converging lenses (or convex lenses). They refract incident parallel rays so that they converge at a point on the opposite side of the lens. The others are thinner at the centre than at the rim. They are known as diverging or concave lenses. They refract incident parallel rays so that they appear to diverge from a point located on the incident side of the lens. Any object viewed through such a lens always appears erect and smaller than when viewed with the unaided eye. A meniscus (meaning crescent-shaped) lens has one convex surface and one concave surface. The principal axis or optical axis of a lens is the line joining the centres of curvature of the two surfaces. Rays making small angles with the optical axis are referred to as paraxial rays. The optical centre of a thin lens is such that any ray passing through that point is not deviated. For the cases we will deal with in this course, the optical centre of a lens can conveniently be thought of as the geometric centre of the lens. Figure 4.1: Different types and shapes of lenses FOCAL LENGTHS There are two points on the optical axis which need special attention. The following figure shows a pencil of paraxial rays from an object at an infinite distance from a converging lens. As such, the pencil will be effectively parallel on reaching the lens. These rays are refracted through the point F2 which is called the second principal focus or second focal point of the lens. The distance from the optical centre to F2 is called the second focal length of the lens and will be referred to by the symbol f2. Figure 4.2: Illustration of the second focal point of a converging lens Similarly, in the next figure, there is the point F1 from which paraxial rays, after refraction by the lens, emerge parallel to the axis. F1 is referred to as the first principal focus or first focal point of the lens, and the distance from the optical centre to F1 is referred to as the first focal length of the lens and is given the symbol f1. F2Thin Lenses I Figure 4.3: Illustration of the first focal point of a converging lens If the lens is thin, and the medium on each side of the lens is the same, then the numerical values of f1 and f2 will be identical. Note that the diagrams show lenses with exaggerated thicknesses and rays that make greater angles with the optical axis than should be the case for paraxial rays. This has been done simply to allow the clear exposition of the refraction at each surface. When “the focal length” of the lens is stated without specifying it to be the first or second focal length, it must always be inferred that this means the second focal length of the lens. In the case of a diverging lens, there are two similar principal foci. The next figure shows the point F1, the first principal focus or first focal point of a diverging lens. Rays directed towards this point emerge from the lens parallel to the optical axis. Once again, the distance from the optical centre to F1 is the first focal length, f1. F1 can be seen to be on the opposite side of the lens compared to the incident light. Figure 4.4: Illustration of the first focal point of a diverging lens Similarly, the second principal focus or second focal point (designated F2) is formed where rays that are incident on a diverging lens parallel to the optical axis appear to diverge from. The distance from the optical centre to F2 is thus the second focal length of the lens. The figure shows that the second focal point lies on the same side as the incident light. As before, if the lens is thin and the media on each side of the lens are identical in refractive index, then the focal lengths have the same magnitude. F1 F1Thin Lenses I Figure 4.5: Illustration of the second focal point of a diverging lens REAL AND VIRTUAL The major usefulness of lenses is their ability to form images of an object. The object may be self-luminous, giving off its own light (like the sun or a light bulb), or it may reflect the light that falls on it (like an apple or a page of this book). In either case, an image of the object is formed where light rays that come from points on the object intersect or at the points from which

OPTOMETRY · SEMESTER 2 Thin lenses 2 Physical and Geometric Optics START READING NOTES Contents of This Topic Thin lenses 2 SUMMARY OF IMAGE FORMATION DIVERGING LENSES THE LENS EQUATION L'−L = F NEWTON’S LENS EQUATION LINEAR MAGNIFICATION POWER OF A LENS LENS SYSTEMS F = F1 + F2 Thin lenses 2 THIN LENSES II CHAPTER CONTENTS SUMMARY OF IMAGE FORMATION………………………………………………………………………………………………………………. 1 THE LENS EQUATION………………………………………………………………………………………………………………………………….. 2 NEWTON’S LENS EQUATION……………………………………………………………………………………………………………………….. 3 LINEAR MAGNIFICATION……………………………………………………………………………………………………………………………… 3 POWER OF A LENS……………………………………………………………………………………………………………………………………… 4 LENS SYSTEMS…………………………………………………………………………………………………………………………………………… 4 LENSES IN CONTACT………………………………………………………………………………………………………………………………….. 4 LENS-MAKER’S EQUATION………………………………………………………………………………………………………………………….. 5 SUMMARY OF IMAGE FORMATION CONVERGING LENSES N.B. When referring to an object position, F should be understood to be the first focal point. Similarly, F should be understood to be the second focal point when referring to an image position. We can use the graphical construction techniques of the previous chapter to analyze the nature and the position of the images obtained for various object positions. The results obtained are as follows.Thin Lenses II 5. When the object is between the lens and the focal point′, the image is formed behind the object. It is virtual, erect and larger than the object. This is how a converging lens can be used as a magnifier. Figure 5.1: Converging lenses: object between lens and focal point B. An object placed at the focal point produces an image at infinity. Figure 5.2: Converging lenses: object at focal point C. When the object is between F and 2F, the image is formed beyond 2F. The image is real, inverted and larger than the object. Figure 5.3: Converging lenses: object between focal point and twice the focal length 2F F F 2F 2F F F 2F F FThin Lenses II If we compare the situations in A and C, we see that in both cases an enlarged image is produced. However, the situation in A is preferred for the use of the lens as a magnifier, because there an ERECT image is produced. D. When the object is at 2F, we find the image at 2F. It is real, inverted and the same size as the object. (See number 5 in Problem Set 4.) Figure 5.4: Converging lenses: object distance at twice the focal length E. An object placed beyond 2F forms an image between F and 2F. It is real, inverted and smaller than the object. Figure 5.5: Converging lenses: object distance further than twice the focal length F. When the object is at infinity, the image is at F. It is again real, inverted and smaller than the object. Recall that this is how the focal point of a converging lens was defined in the previous chapter. DIVERGING LENSES In the case of a converging lens, the image is sometimes real and sometimes virtual. In a diverging lens, the image is always virtual, erect and diminished. The image is situated between the object and the lens. 2F F F 2F 2F F F 2FThin Lenses II Figure 5.6: Diverging lenses, for all object positions THE LENS EQUATION Instead of carrying out ray-tracing for each situation, we can determine the nature and position of an image by calculation instead. A simple equation relates the positions of the image and the object of a thin lens to the lens’s focal length, f. o i + = d df 1 11 Where do is the position (distance) of the object, di is the position of the image and f is the focal length. We can rewrite the equation in terms of vergence as L'−L = F L is the object vergence L’ is the image vergence F is the focal power of the lens Sign Convention Converging lenses have a positive focal length and diverging lenses a negative focal length. F FThin Lenses II NEWTON’S LENS EQUATION Newton showed that the conjugate points O and I obey the relation: x1x2 = f2 This formula states that the product of the distances of two conjugate points from the respective foci of a lens or mirror is equal to the square of the focal length. From this, it follows that x2 decreases as x1 increases. This implies that the image recedes from the focus on the right (away from the lens) when the object approaches the lens. This can be confirmed with the ray diagrams at the start of this chapter. Figure 5.7: Newton’s lens equation LINEAR MAGNIFICATION As previously stated in chapter 4, the linear magnification (m) of an optical system is the ratio between the size of the image and that of the object. A magnification of exactly 1 means that the image and the object are the same size. By “size” we mean any linear dimension such as height or width. Image Size (I) Object Size (O) m = A relationship can also be derived relating magnification to image and object distances: i o d d = = Image Size (I) m – Object Size (O) If we use the sign convention specifying that distances above the optical axis are considered positive and that distances below are considered negative, we obtain the following result: If the image is erect with respect to the object, the magnification is positive. If the image is inverted with respect to the object, the magnification is negative. The magnification is sometimes referred to as the relative size. Thin Lenses II POWER OF A LENS The power of a lens, F, is defined as the reciprocal of the focal length, in metres. The unit of power, m-1 , will be recognized as being the same as the unit of vergence, the dioptre (D). The power of a lens is thus its ability to change the vergence of a pencil of rays. The fact that vergence at a point and the power of a lens share the same units, does NOT mean that the vergence at a point equals the power of a lens. Expressed in symbols: 1 F = f LENS SYSTEMS In general, when light passes through more than one lens,

OPTOMETRY · SEMESTER 2 Reflection Physical and Geometric Optics START READING NOTES Contents of This Topic Reflection REFLECTING SURFACES REFLECTION AT PLANE SURFACES LAWS OF SPECULAR REFLECTION IMAGE FORMED BY A PLANE MIRROR ROTATION OF REFLECTED RAY MULTIPLE IMAGES REFLECTION AT CURVED MIRRORS IMAGE FORMATION BY SPHERICAL MIRRORS THE MIRROR EQUATION USES OF MIRRORS Reflection REFLECTION CHAPTER CONTENTS REFLECTING SURFACES…………………………………………………………………………………………………………………………….. 1 REFLECTION AT PLANE SURFACES ……………………………………………………………………………………………………………. 0 LAWS OF REGULAR REFLECTION……………………………………………………………………………………………………………….. 1 IMAGE FORMED BY A PLANE MIRROR ………………………………………………………………………………………………………… 1 ROTATION OF REFLECTED RAY………………………………………………………………………………………………………………….. 2 MULTIPLE IMAGES………………………………………………………………………………………………………………………………………. 2 REFLECTION AT CURVED MIRRORS …………………………………………………………………………………………………………… 4 IMAGE FORMATION BY SPHERICAL MIRRORS…………………………………………………………………………………………….. 6 THE MIRROR EQUATION……………………………………………………………………………………………………………………………… 6 USES OF MIRRORS …………………………………………………………………………………………………………………………………….. 7 REFLECTING SURFACES A ray of light may be turned through an angle by being reflected at a suitable surface. (Bear in mind that some reflection occurs together with any refraction.) Highly polished glass will reflect only approximately 4% of the incident light near to normal incidence, whereas highly polished silver reflects about 95% of the incident light. Many other reflecting surfaces may be used (even obsidian!), but in all cases, some loss of energy occurs. The original optical reflecting surface was a chemical deposit of silver on glass. The silver was on the front surface (front-reflecting mirror), in contrast to reflectors produced by depositing a layer of aluminium or tin amalgam on the back of a sheet of glass (back-reflecting mirror).Reflection One of the main drawbacks to using such back-reflecting mirrors is the formation of multiple images. For this reason, back-reflecting mirrors are never used in high-grade optical equipment and instruments. The glass used for front-reflecting mirrors need not be of the best quality because the light does not actually pass through it. The glass is simply a carrier for the reflecting surface. However, the deposit may not be able to be protected against corrosion and may have to be renewed at frequent intervals. REFLECTION AT PLANE SURFACES All real objects reflect a certain proportion of the light falling upon them, and it is this reflected light that enables us to see them. In most cases, the surface of the object has irregularities that spread out an initially parallel beam of light in all directions to produce diffuse reflection. Figure 6.1: Irregular surface causing diffuse reflection A surface so smooth that any irregularities in it are small relative to the wavelength of light falling on it behaves differently. When a parallel pencil of rays of light is incident on such a surface, it is reflected in a single definite direction. Such reflection in one definite direction is termed regular or specular reflection. Figure 6.2: Regular surface causing specular reflection For instance, consider the two types of reflection from a road surface that one sees while driving at night. When the road is dry, light from oncoming vehicles is scattered off the road in different directions (diffuse) and the road is quite visible. This diffuse reflection is also responsible for the perception of depth which makes the raised road markings visible. On a rainy night, when the road is wet, the road irregularities are filled with water. Because the wet surface is quite smooth, the light undergoes specular reflection. This means that the light is reflected straight ahead and the driver only sees what is directly in front. Our concern in this course is only with this specular reflection.Reflection LAWS OF SPECULAR REFLECTION MIRROR Figure 6.3: Incident and reflected angle on a mirror surface This diagram shows light being reflected from the surface of a smooth plane mirror. Note, once again, that the angles are measured between the ray and the normal. Law 1: The angle of incidence is equal to the angle of reflection. Law 2: The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane. One of the consequences of these laws is a common occurrence in photographs of individuals – their eyes appear to be glowing red. This occurs when a flash unit is used and the unit is very close to the camera lens. The light from the flash unit will enter the eye normally and will thus be reflected back along its original path from the retina. This reflected light can enter the lens. Most of the light reflected from the retina is red, mainly due to the blood vessels at the back of the eye, giving the “red-eye effect”. IMAGE FORMED BY A PLANE MIRROR The image formed by a plane mirror can be analyzed by graphical construction in a similar fashion to what we did with lenses. The results of this analysis give the characteristics of the image formed by a plane mirror. The image is: 1. as far behind the mirror as the object is in front. 2. the same size as the object. 3. virtual. 4. erect. 5. laterally inverted. Figure 6.4: Image formed by a plane mirror i r NormalReflection Lateral inversion refers to the phenomenon that the image is reversed from left to right in comparison to the object. For example, when you look in a mirror and touch your right ear, it is the left ear being touched in the image. Figure 6.5: Lateral inversion in mirrors ROTATION OF REFLECTED RAY If a ray is incident on a plane mirror at an angle i, the angle of reflection is also equal to i. The angle between the two rays will thus be 2i. Suppose now that the mirror is rotated through an angle θ. The angle of incidence will increase by θ and so will the angle of reflection. The total angle between the two rays is now 2i+ 2θ. Consequently, the angle of reflection will rotate through an angle of 2θ. Thus, the reflected ray rotates through twice the angle through which the mirror rotates. This important effect may be used as a magnifying device for small rotations in applications known as optical levers. MULTIPLE IMAGES With a thick back-silvered plane mirror, some light is always reflected at

OPTOMETRY · SEMESTER 2 Interference Physical and Geometric Optics START READING NOTES Contents of This Topic Interference INTERFERENCE COMPLETE CONSTRUCTIVE INTERFERENCE. YOUNG’S DOUBLE-SLIT EXPERIMENT Bright Fringes Dark Fringes CHANGE OF PHASE DUE TO REFLECTION EXAMPLES ANTI-REFLECTION COATINGS ANTI-REFLECTIVE COATINGS ON LENSES MATERIAL REFRACTIVE INDEX Interference INTERFERENCE CHAPTER CONTENTS INTERFERENCE ………………………………………………………………………………………………………………………………………….. 1 YOUNG’S DOUBLE-SLIT EXPERIMENT…………………………………………………………………………………………………………. 1 INTERFERENCE IN THIN FILMS……………………………………………………………………………………………………………………. 4 ANTI-REFLECTION COATINGS …………………………………………………………………………………………………………………….. 7 INTERFERENCE When two or more waves arrive at a point in a medium simultaneously, the principle of superposition can be used to find the resultant displacement. The superposition principle states that when two or more waves traverse the same space, the net amplitude at each point is the sum of the amplitudes of the individual waves. This interaction of waves that exist at a point at the same time is known as interference. The same holds for electromagnetic waves moving through a medium. The amplitudes can add together either constructively or destructively. In constructive interference, the amplitude of the resultant wave is greater than that of either of the individual waves, while in destructive interference, the resultant amplitude is less than that of either of the individual waves. In the case of two waves of equal amplitude, complete constructive and complete destructive interference may occur and will be illustrated first (figure 7.1). Fundamentally, all interference associated with light waves arises as a result of combining the fields that constitute the individual waves.Interference COMPLETE CONSTRUCTIVE INTERFERENCE. COMPLETE DESTRUCTIVE INTERFERENCE. INTERFERENCE DUE TO WAVES PARTIALLY IN PHASE. Figure 7.1: Constructive and destructive interference Interference effects in light waves are not easy to observe because of the short wavelengths involved. These short wavelengths mean that they have very high frequencies and therefore there are many waves interacting per second. Interference In order to observe sustained interference in light waves from more than one source, the following conditions must be met: 1. The sources must be coherent, that is, they must maintain a constant phase with respect to each other. 2. The sources must emit identical wavelengths. (This condition is easiest to meet if the sources are monochromatic.) If two separate light sources are placed side by side, no interference effects are observed because in this case the light waves emitted by each of the sources are emitted independently of the other source. Hence, their emissions do not maintain a constant phase relationship with each other over the time of observation. Light from an ordinary light source undergoes such random changes about once every 10-8 seconds. Therefore the conditions for constructive interference, destructive interference, or some intermediate state last for times of the order of 10-8 seconds. The result is that no interference effects are observed since the eye cannot follow such short-term changes. Such light sources that do not maintain a constant phase relationship are said to be incoherent. Please note that interference phenomenon can be observed with incoherent “white” (i.e. polychromatic) light (e.g. colour patterns on soap bubbles, anti reflections coatings on spectacles, etc.) but the conditions for them to be observed are much more strict (and difficult to put in place) than with coherent light. YOUNG’S DOUBLE-SLIT EXPERIMENT The phenomenon of interference in light waves from two sources was first demonstrated by Thomas Young in 1801. Figure 7.2: Young’s double slit experimentInterference Sunlight is incident on a screen, which is provided with a narrow slit S0. The waves emerging from this slit arrive at a second screen, which contains two narrow, parallel slits, S1 and S2. Since these slits are equidistant from the first slit, they serve as a pair of coherent light sources because waves emerging from them originate from the same wavefront and therefore maintain a constant phase relationship. Furthermore, they emit identical wavelengths since the waves arise from the same source. In this way the conditions for sustained interference are met. The light from the two slits produces a visible pattern on a target screen. The pattern consists of a series of alternating bright and dark parallel bands called fringes surrounding a fairly broad central light band. Young accounted for the interference pattern generally as follows: When the light from slits S1 and S2 arrives at a point on the target screen such that constructive interference occurs at that location, a bright line appears. When light from the two slits interferes destructively at any location on the screen, a dark line results. Two waves (that necessarily leave the slits in phase) strike the screen at the central spot. Since they travel an equal distance, they arrive in phase and as a result complete constructive interference occurs here and a bright spot is observed. Figure 7.3: Constructive interference in Young’s experiment Figure 7.4 illustrates how the first dark band away from the central spot is formed. The waves leaving the two slits once again start out being in phase but travel different distances. As a result, one wave is half a wavelength behind the other so that the crest of one wave overlaps with the trough of the other. Complete destructive interference results and a dark fringe is observed.Interference Figure 7.4: Destructive interference in Young’s experiment In the following diagram, the waves again start out in phase and travel different distances, but the difference in path length is exactly one wavelength and the waves thus reach the screen in phase. Complete constructive interference results and another (higher order) bright band is observed. Figure 7.4: Constructive interference of higher order in Young’s experiment The foregoing can be summarized as follows: If the path difference is either zero or some integral multiple of the wavelength, the two waves are in phase at the screen and a bright fringe results. When the path difference is an odd multiple of λ/2, the two waves arriving at the screen will be 180° out of phase and a dark fringe will result. What is the effect of wavelength on the interference pattern? The shorter the wavelength of the incident light, the closer the fringes are to each other, and the

OPTOMETRY · SEMESTER 2 Photometry Physical and Geometric Optics START READING NOTES Contents of This Topic Photometry INTRODUCTION SOLID ANGLES LUMINOUS POWER AND LUMINOUS INTENSITY LUMINOUS EFFICIENCY AND LUMINOUS EFFICACY RADIOMETRIC AND PHOTOMETRIC UNITS ILLUMINANCE FUNDAMENTAL LAWS OF PHOTOMETRY Law 2. LIGHT SOURCES GAS DISCHARGE TUBES Low Pressure Sodium Lamps High Pressure Sodium Lamps FLUORESCENT LAMPS LASERS STIMULATED EMISSION LASER ACTION MEDICAL APPLICATIONS LASER SAFETY LIGHT EMITTING DIODES HOW AN LED WORKS ADVANTAGES DISADVANTAGES Photometry PHOTOMETRY CHAPTER CONTENTS INTRODUCTION………………………………………………………………………………………………………………………………………………… 2 SOLID ANGLES…………………………………………………………………………………………………………………………………………………. 2 LUMINOUS POWER AND LUMINOUS INTENSITY……………………………………………………………………………………………….. 3 LUMINOUS EFFICIENCY AND LUMINOUS EFFICACY…………………………………………………………………………………………. 4 RADIOMETRIC AND PHOTOMETRIC UNITS……………………………………………………………………………………………………….. 4 ILLUMINANCE …………………………………………………………………………………………………………………………………………………… 4 FUNDAMENTAL LAWS OF PHOTOMETRY …………………………………………………………………………………………………………. 5 LIGHT SOURCES………………………………………………………………………………………………………………………………………………. 6 LASERS ………………………………………………………………………………………………………………………………………………………….. 10 LIGHT EMITTING DIODES………………………………………………………………………………………………………………………………… 13Photometry INTRODUCTION Photometry may be described as the study and measurement of light in terms of the visual response it produces. The eye does not respond with equal sensitivity to light of different colours; the sensitivity is a maximum for green light and it decreases towards both ends of the visible spectrum. Consequently, although they produce significant damage to the eye, IR and UV light do not produce that much of a visual response compared to some visible light. The sensitivity curve, sometimes referred to as the luminosity curve, gives the relative brightness as assessed by the average eye of the different colours of the spectrum. For the average light adapted eye at moderate intensities (photopic vision) the maximum visual effect is obtained with light of wavelength 555nm (yellow-green). This maximum shifts to 500nm in scotopic vision (for the average dark adapted eye). This is known as the Purkinje shift. The electromagnetic spectrum covers a wide range of wavelengths. However, in photometry we restrict ourselves to visible light (officially the region from 360 to 830 nm). This is done because the aim of photometry is to measure light in such a way that the results correlate with human vision. (The study of radiant energy, including visible light, without regard to its visual response is known as radiometry.) Occupational Safety legislation puts a duty on employers to provide sufficient lighting of suitable standard for specific purposes. By ensuring that photometric quantities conform to standards, mankind benefits in terms of: reduction of eyestrain fewer accidents better working conditions greater productivity improved leisure facilities etc. Most practical light sources are incandescent or fluorescent sources which emit energy over a wide range of wavelengths, mainly as wasted heat in the infrared part of the spectrum. The distribution of luminous power is not uniform in all directions and light is emitted from different points on the source. However, we simplify matters by assuming that: (i) the dimensions of the source are negligible compared to its distance from a surface or object and (ii) the light is emitted equally in all directions. The first assumption amounts to saying that we will consider all sources to be point sources. The second assumption is of course equivalent to saying that we consider all sources to be isotropic. SOLID ANGLES The unit solid angle, or steradian, is the solid angle of a cone that, having its apex at the centre of a sphere, cuts off an area of the sphere’s surface equal to the square of its radius. Photometry Figure 8.1: Representation of a steradian The number of steradians in any solid angle is given by the area of the sphere’s surface that is included in the angle divided by the square of the radius, i.e.: 22 area Area at cone opening solid angle = = r (cone length) LUMINOUS POWER AND LUMINOUS INTENSITY The term central to photometry is luminous power, Φ. This is because the visual sensation produced depends on the rate at which light energy reaches the retina and not on the total energy. Hence, a dimly lit object will not continue to grow brighter and brighter as it is observed. The rate at which light energy flows is called the luminous power (flux). The luminous power is therefore the luminous energy flowing per second and this could be expressed in joules per second (watts). However, in photometry, we use the unit called the lumen. Definition: One lumen is the luminous power emitted into a unit solid angle (1 steradian) from a point source of intensity 1 candela. The total number of lumens emitted in all directions from a source is thus the luminous power. If we consider the luminous power emitted per unit solid angle, we are now dealing with the luminous intensity. This is indicated by the symbol I and has as unit the candela (cd). We saw previously that there are 4π steradians round a point source. Therefore, I is related to Φ by the relationship : (Φ is the Greek letter phi. ) Φ = 4πIPhotometry LUMINOUS EFFICIENCY AND LUMINOUS EFFICACY A light may produce a great deal of radiant energy, (energy in the form of electromagnetic radiation) but relatively little luminous energy (visible light). To be useful for lighting purposes, we need most of the output to be in the visible region. Two concepts are used to indicate the effectiveness of a source. The luminous efficiency indicates the fraction of the total power produced which is actually visible. The units will be lumens per lumen. luminous flux emitted Luminous efficiency = total radiant power emitted For example, a 60 watt incandescent light bulb produces the same luminous power as a 15 W fluorescent bulb. This is because much of the incandescent light bulb output is invisible infrared. Its luminous efficiency is much less since it is converting a large portion of the power input into wasted heat. It should be noted that the power output at each wavelength is multiplied by a factor determined by the visual response of the eye to light of that wavelength. The human eye is most sensitive to green light with a wavelength of 555 nm. One watt of such light is “worth” 683 lumens. In contrast, infrared and ultraviolet radiation are invisible and therefore count zero

OPTOMETRY · SEMESTER 2 Optical instruments Physical and Geometric Optics START READING NOTES Contents of This Topic Optical instruments INTRODUCTION CONTROL OF LIGHT IN THE CAMERA SPEED OF A CAMERA LENS DEPTH OF FIELD COMPARISON OF THE CAMERA TO THE EYE LENS ABERRATIONS CHROMATIC ABERRATION THE SIMPLE MAGNIFIER YOU ARE NOT REQUIRED TO KNOW THESE DERIVATIONS. THE COMPOUND MICROSCOPE THE TELESCOPE REFLECTING TELESCOPE TERRESTRIAL TELESCOPE Optical instruments OPTICAL INSTRUMENTS CHAPTER CONTENTS INTRODUCTION…………………………………………………………………………………………………………………………………………… 1 THE CAMERA………………………………………………………………………………………………………………………………………………. 1 LENS ABERRATIONS…………………………………………………………………………………………………………………………………… 5 THE SIMPLE MAGNIFIER……………………………………………………………………………………………………………………………… 8 THE COMPOUND MICROSCOPE…………………………………………………………………………………………………………………11 THE TELESCOPE ……………………………………………………………………………………………………………………………………….13 INTRODUCTION An understanding of the optics of basic optical instruments is important since the applications are to be found in many different clinical instruments as well as the therapeutic devices that are used by practicing optometrists. THE CAMERA CONSTRUCTION The single-lens photographic camera is a simple optical instrument. It consists of a light-tight box, a converging lens that produces a real image, and a film behind the lens to receive the image. Unlike the eye, this type of camera lens has a fixed focal length. Focusing is accomplished (where possible) by varying the distance between lens and film. For proper focusing, which leads to sharp images, the lens-to-film distance will depend on the object distance as well as on the focal length of the lens. Optical instruments The shutter, located behind the lens, is a mechanical device that is opened for selected time intervals. To take a photograph, you first adjust the lens position so that a real, inverted image of the object is in focus in the plane of the film. Then, when you press a button, the shutter momentarily opens and an image forms on the film. This image is stored in the light-sensitive material for later chemical processing, yielding a reproduction of the scene. Digital cameras replace the photographic film with an electronic sensor array, the sensitivity of which can be adjusted to resemble the response of film. The effective lens opening is controlled by an iris diaphragm, the size of which can be varied. It is given this name because it performs a similar function to the iris of the eye. In some cameras, the iris diaphragm also acts as the shutter; while normally closed, the iris opens to a pre-determined size in order to exposure the film. CONTROL OF LIGHT IN THE CAMERA With modern photographic films and with adequate lighting, the shutter only needs to be open for a small fraction of a second to successfully record an image on film. Many cameras have a range of available shutter speeds – that is, lengths of time during which the shutter is open. Shutter speeds of 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, and 1/2000 second are standard. Note that each of these is approximately half as long (or, in photographic terminology, “twice as fast”) as the preceding one. The faster the shutter speed is, the faster the object can be moving and still produce a sharp image. With this arrangement, moving objects can be photographed with the use of short exposure times, and dark scenes (low light levels) with the use of long exposure times. Without this control, it would be impossible to take stop-action photographs. For example, a speeding race car would move far enough while the shutter was open to produce a blurred image. Stationary objects can be shot with a shutter speed of 1/60 s since there is no chance of motion blurring, although the camera must be held steady. Older cameras had semi-circular shutters, but later models usually had diaphragm shutters. The amount of light reaching the film must be carefully controlled to avoid underexposure (too little light for any but the brightest objects to show up) or overexposure (too much light, so that all bright objects look the same, with a consequent lack of contrast). To control the exposure, a “stop” or iris diaphragm, whose opening is of variable diameter, is placed behind the lens. The size of the opening is varied to compensate for different lighting conditions, the sensitivity of the film used, and for different shutter speeds. Getting the “correct exposure” in a photograph corresponds to allowing the appropriate amount of light to strike the film. Therefore “correct exposure” means the correct luminous energy per unit area of the film. This amount is different for different types of film. The amount of light that strikes the film is determined not only by how long the shutter stays open, but also by how large the effective lens opening is. Thus it is analogous to the amount of water flowing from a tap, which depends on both the length of time the tap stays open and the cross-sectional area of the opening. We can deduce a relationship between “amount of light” and camera parameters as follows: (See the photometry section of chapter 8 for detail on terms used here.) 1. The amount of luminous power falling on the image in a camera is proportional to the area of the lens aperture, or to d2 , where d is the diameter of the aperture. This follows from the fact that the area of a circle is given by πr 2 (πd2 /4). 2. The area of the image formed is proportional to f2 , where f is the focal length of the lens, since the length of the image formed is proportional to the focal length. It therefore follows that the luminous power per unit area of the image, or illuminance of the image, is proportional to d2 /f2. The time of exposure (shutter speed), t, for activating the chemicals on the given negative is inversely proportional to the illuminance (the greater the amount of light falling on unit area per unit time, the less time will be needed to get the “correct amount of light” onto the film). Hence: t∝ f 2 /d2 OR t ∝ (f/d)2Optical instruments The size of the lens opening, (often referred to as the aperture or relative aperture), is usually measured by what is called

OPTOMETRY · SEMESTER 2 Diffraction and polarisation Physical and Geometric Optics START READING NOTES Contents of This Topic Diffraction and polarisation DIFFRACTION POLARISATION Figure 10.2: Polarisation POLARISATION BY SELECTIVE ABSORPTION POLARISATION BY REFLECTION POLARISATION BY DOUBLE REFRACTION Tourmaline 1.669 1.638 POLARISATION BY SCATTERING Diffraction and polarisation DIFFRACTION AND POLARISATION CHAPTER CONTENTS DIFFRACTION ……………………………………………………………………………………………………………………………………………… 1 POLARISATION……………………………………………………………………………………………………………………………………………. 0 POLARISATION BY SELECTIVE ABSORPTION ……………………………………………………………………………………………… 1 POLARISATION BY REFLECTION…………………………………………………………………………………………………………………. 3 POLARISATION BY DOUBLE REFRACTION ………………………………………………………………………………………………….. 6 POLARISATION BY SCATTERING ………………………………………………………………………………………………………………… 7 DIFFRACTION Think back to Young’s double-slit experiment. If the light truly travelled in straight-line paths after passing through the slits, the waves would not overlap and no interference pattern would be seen. In other words, the light deviates from a straight-line path and enters the region that would otherwise be shadowed. This deviation of light from its initial line of travel (in the same medium) is called diffraction. It can also be referred to as the bending of light round the edges of an obstacle. When light passes through an opening that is large compared to the wavelength of light, it casts a shadow, as shown on the left in figure 10.1. But if we pass light through a thin slit we see that the light diffracts. The sharp boundary between the light and the dark area disappears, and the light spreads out like a fan to produce a bright area that fades into darkness without sharp edges. This can be seen on the right below. Diffraction and polarisation Figure 10.1: Representation of the principle of diffraction Diffraction is not confined by narrow slits or to openings in general but can be seen for all shadows. On close examination, even the sharpest shadow is blurred slightly at the edge. Under ordinary conditions, we seldom notice the diffraction of light. Light sources such as incandescent lamps or the sun are not monochromatic point sources, and the diffraction patterns due to different parts of the source and to different wavelengths usually overlap and obscure each other. Diffraction is a problem when we wish to see very small objects with microscopes. If the size of the object is about the same as the wavelength of light, the image of the object will be blurred by diffraction. If the object is smaller than the wavelength of light, no structure can be seen. The entire image is lost due to diffraction. No amount of magnification or perfection of microscope design can defeat this fundamental diffraction limit. One of the goals of an optical design is to achieve resolution as close to the theoretical diffraction limit as possible. This is true not only of clinical instruments, but also corrective lenses. Larger objects viewed with visible light can be seen clearly. A similar situation occurs with very small objects if the wavelength of the waves used to view the object is made very small. This can be done by using a beam of electrons instead of a light beam. POLARISATION An ordinary beam of light consists of a large number of waves emitted by the atoms or molecules of the light source. Each atom produces a wave with its own orientation of E, the electric field vector, corresponding to the direction of atomic vibration. However, because all directions of vibration are possible, the resultant electromagnetic wave is a superposition of waves produced by the individual atomic sources. The result is an unpolarised light wave. The direction of polarisation of the electromagnetic wave is defined to be the direction in which E is vibrating.Diffraction and polarisation LEFT: Light waves having all possible orientations of E, RIGHT: Light waves having only ONE orientation of E Figure 10.2: Polarisation Note that all directions of the electric field vector are equally probable and all lie in a plane perpendicular to the direction of propagation. At any given point and at some instant of time, there is only one resultant electric field, which can be resolved into a vertical and a horizontal component. You should not be misled into thinking that the figure is showing that the electric field vector has a number of directions at a particular time. A wave is said to be linearly polarised if E vibrates in the same direction at all times at a particular point. Such a wave is described as plane-polarised, or simply polarised. The phenomenon of polarisation provides firm evidence of the transverse nature of electromagnetic waves. All waves show interference and diffraction but only transverse waves can be polarised. It is possible to obtain a linearly polarised beam from an unpolarised beam by removing from it all components except those whose electric field vectors oscillate in a single plane. We shall now discuss some physical processes for producing polarised light from unpolarised light. POLARISATION BY SELECTIVE ABSORPTION The most common technique for obtaining polarised light is to use a material that transmits waves whose electric field vectors vibrate in a plane parallel to a certain direction and absorbs those waves whose electric field vectors vibrate in Diffraction and polarisation other directions. Any substance that has the property of transmitting light with the electric field vector vibrating in only one direction is called a dichroic substance. Examples of these are tourmaline, and quinine iodosulfate. Tourmaline has the disadvantage of producing coloured polarised light. In 1932, E. H. Land prepared the first version of Polaroid by embedding a thin layer of quinine iodosulfate in a plastic sheet and stretching the plastic. An improved version followed in 1938 which used long polymeric molecules of polyvinyl alcohol that polarized light through selective absorption by oriented molecules. The sheets are stretched during manufacture so that the long-chain molecules align. After such a sheet is dipped into a solution containing iodine, the molecules become conducting. However, the conduction takes place primarily along the hydrocarbon chains since the valence electrons of the molecules can move easily only along the chains (recall that valence electrons are “free” electrons that can readily move through the conductor). As a result, the molecules readily absorb light