OPTICAL INSTRUMENTS

Simple Microscope

The Structure of the Simple Microscope Describe the structure of the simple microscope A magnifying gless, an ordinary double convex lens with & short focal length, is @ simple microscope. The reading lens and hand lens are instruments of this type. When an object is placed neuter such a lens than its principal focus, ie., within its focal length, an image is produced that is ereet and larger than the original object. The image is also virtual; i, it eannot

be projected on a screen as cana real image The Mode of Action of a Simple Microscope Describe the mode of action of a simple microscope The image formed by magnifying glass or simple microscope is virtual and erect object place between prineipal focus (f) and convex lens “ i f, Ls =< ie i ee observer iy SS

D Se

Using the equation of tens (Lens formula), wu+tV=VF Adopting the ‘real is positive’ sign convention we obtain:

WU-0V =F

25F

The above formula shows the means of obtaining the distance of object, U. Magnication (M) of simple microscope

M= Image distance, V

Object distance, U

M=via ) so

Insert eqa (if) into (i)

M= V1 (25046425)

Example 1

AA simple microscope with lens of focal length Sem is used to read division of 2 seale 0 Smm in size, How large will the division be seen through the simple microscope? Data given

• Focal length, f= Sem
• Required to find magnification, M

Soln: From

The magnification of lens ~ 6 Let the size of the object be ho and that of the image be hi. Then:

st

Hence, each division will appear to have a size of 3.0mm viewed through the simple microscope

A Simple Microscope

Construct a simple microscope Parts of simple microscope C—_—_——_.}

Compound Microscope

‘The Structure of a Compound Microscope Describe the siructure of a compound microscope A compound microscope is an optical instrument used to produce much grester magnification than that produced by simple microscope. The main features of a compound microscope includes two short-focus convex lenses, the objective lens, and the eyepiace Demonstration tetas Tt \, ae os rath. + {

PSE TT

h ome S Le ‘The Mode of Action of a Compound Microscope Describe the mode of action of a compound microscope ‘The most commonly used microscope for general purposes is the standard compound microscope. It magnifies the size ofthe object by a complex system of lens arrangement Iv has a series of two lenses; (i) the cbjective lens close to the object o be observed and (ii) the ‘ocular lens or eyepiece, through which the image is viewed by eye. Light from a light source

(onirtor or electric lamp) passes through a thin transparent object. The objective lens produces a magnified ‘real image’ (first image of the object). This image is again magnified by the ocular lens (eyepiece) to obtain a magnified “virtual image’ (final image), which can he seen by eye through the eyepicee. As light passes directly from the source to the eye through the two lenses, the field of vision is brightly illuminated, That is why it is a bright

Feld microscope. ‘The Magnification of a Compound Microscope Determine the magnification ofa compound microscope The object lens forms a real and inverted image Lof the object O ( the image is slightly magnified), The eyepiece lens acts as a magnifying glass forthe first image TT and produces a magnifical virtual image.

The object is placed just beyond the principal (Fo) of the objective lens so that thatthe eal image . is formed inside the principal focus (F) of the eye piece. The eyepiece treats the real image I 4s an object and then forms its magnified virtual image 12 compound microscope to the object distance. The megnification produced by objective lens is Vis the image distance The magnification given by the eyepiece is given by.

Me= Mame

Combine eqn (i) and (i) Then

The above formula shows thatthe final virtual image is formed at the east distance of distinet

Uses of'a Compound Microscope

Te uses of compound microscope includes the following

• Used to magnify microorganism such as bacteria which cannot be seen by naked eyes
• Used in hospitals widely to detect microorganisms in specimens provided by patients. A

Example 2

sa A certain microscope consists of two converging lenses of focal length 1Oem and 4em for the objective and eyepiece, respectively. The two lenses are separated by a distance of 30cm. The instrument is focused so that the final image is at infinity. Calculate the position of the object and the magnification of the objective lens, For the objective lens

UU+IV= Eo

Where

The objective lons forms a real image of the object at the principal focus of the eyepiece Thus

The magnification given by the objective lense is given by Whereas:

Us 16.25em_

Astronomical Telescope

The Structure of an Astronomical Telescope An Astronomical ‘Ielescope is used for observing heavenly bodies like stars and planets (generally bodies which are very far away from normal vision of human eyes ). Like compound microscope, it consists of two convex lenses, objective lens and the eyepicee The focal length Fb of the objective lens is longer than the focal length Fe of the eye piece lens Rays of light from a distant object are nearly parallel when they strike the objective lens of

distant object is in the focal plane The eye piece forms the final magnified image at infinity \When the telescope i adjusted in such a way that the final image is at infinity i is said to be in isthe maximum separation between the objective lens and the eyepisee lens.

‘The Mode of Action of an Astronomical Telescope Describe the made of action of an astronomical relescope The main reason for a distant object to be smaller is thatthe two objects subtend different angles at the eye. In other words, we can say that different angles substended by the eye causes a distant object to appear small Pad 7 os eel The object AB and CD are of the Same height. The object CD is closer to the eye than AB.

The object CD appears to be taller than AB because angle B that CD subtends at the eye is seater than the angle x subtended by AB at the eye. Images there can be made to appear large by bringing them closer to the eye Ina telescope the final image is magnified because it subtends a much greater angle at the eye than does a distant object observed without a telescope. B is the angle subtended by the final image at the eye and X is the angle subtended by a distant object.

The Magnification of an Astronomical Telescope Determine the magnification of an astronomical telescope The magnification of a telescope is defined as the ratio of the angel B (in radians) subtended by the final image at the eye to the angle X subiended by a distant object at the eye Thus, for telescope the magnification is given by: M=Bx i From figure B= IQ. ii

X= IQA tit

But Insert eqa (i) and (ii) into aga (i)

(IQMA)

s7

M-=(IAID)

But IA ~ fo and IF ~fe M= forte (so Where

Example 3

fois the focal length of two thin converging lenses of focal lengths 25em and 4em respectively. It is focused on the moon which subtends an angle of 0.6° at the objective lens. The final image is formed at the observers least distance of distinct vision (25cm in front of the eyepiece). Find the diameter of this image In the previous figure: X=hifo Where fo is the focal length of the objective lens

Where X is the angle in radians subtended at the objective lens by the moon.

The height of the image, h~ 0.2619m The distance ofthis image from the eyepiece is obtained from the relation:

+ WF IV=Wie=dom

• W-125=%
• = (4+ 125)
• (UU)-1 = 25 +4) -1/100
• 100U= (10029)

The magnification, m of the Jens

• M=Viw
• M=(25CM/100/29)
• Ma 29/4

Let the height of the final image of the moon be h

• M=Hih
• hl=mh
• HI= (29/4) (0.2619)
• HE 1.9m

Hence The diameter of the final image of the moon will be 1.90cm Observation of the universe today are best made from the Hubble Telescope, Outside the Earth's atmosphere, this telescope suffer from less interference.

Uses of an Astronomical Telescope

Mention uses of an astronomical telescope Astronomers use telescopes because they're much better than our eyes. Here are a few reasons: 1, Telescopes see lots of colours – telescopes can collect light that our eyes are unable to: radio, microwave, infrared, ultraviolet, x-rays and gamma rays.

2 Telescopes collect lots of light – our pupils are only a few millimeters across, so we can only collect photons over tiny area whereas telescopes can collect photons of huge areas (e.g. a football fickds worth for radio telescopes) we ean only sce details about the same angular size as Jupiter's width. Telescopes ean allow us to

• Telescopes can record observations with cameras – You can see things with your eye and

draw them, but telescopes can share observations with the world! This is especially important for convincing skeptics that what you saw was teal!

A Simple Astronomical Telescope

Projection Lantern

The Structure of the Projection Lantern The projection lantem forms images of slides or camera film onto a distant screen, ‘The film or slide to be projected is inverted snd highly illuminated.

‘The Mode of Action of a Projection Lantern Desoribe the mode of action of a projection lantern

• The slice or film to be projected is inverted and highly illuminated.
• The concave mirror helps to concentrate the light which would otherwise be partly

wasted +The lamp is placed at the principal focus of the concave mirror

• The heat filter reduces the heat at falling on the slide or film so as to avoid it overheating
• Since the image of the projection lantem is Highly magnified, it would not be very bright

if there was not enough illumination

• The condenser directs @ maximum amount of light from the souree of the slide and

produce uniform illumination the sereen. (The condenser is a double in order to reduce chromatic aberration)

• The projection lens forms the image of the slide on the screen.
• The light source is usually located at a distance of 2f from a condenser and invited so that

the image on the screen is upright (erect)

• The focal length of the projection lens is ABOUT TWICE THE FOCAL length of the

‘condenser since the sereen is usually far from the lens, ‘The Magnification of a Projection Lantem Determine the magnification of a projection lantern

Example 4

A lantern projector using a slide of (2em x 2em) projects a picture (lem x lem) onto a screen 12m ftom the projection lens. How far from the lens must the slide be? Find the approximate focal lenath of the projection lens.

ot M=Vvit i

• His the size of image

. His the size of object Thus M=Hi ii

• (u!200)= (2/100)
• U=@/100) (1200)
• U=2em

Uses ofa Projection Lantern

. used in searchlights and headlights,

• sed in physical experiments such a projection ofa spectrum,

A Simple Projection Lantern

Construct a simple projection lantern

Projection Lantern

amg I

The Lense Camera

The Structure of the Lens Camera Lens camera is an instrument which produces an image of object onthe sereen using light. The The optical sytem of the camera are very similar to that of the lantern projector but with the direction of light reversed:The converging lens forms a real image of the object to be photographed (This image is diminished (smaller than the object and inverted) The leas can be moved back and forward with the help of focusing any so that objects at

Aifferent distances can be brought tothe focus. forced image is locate on the film or plate when the shuttled is open for a suitable amount of time as determined by the shutter speed Light enters the camera Box and makes a picture of the object on the film “( The film is sensitive volishn) cy The camera is equipped with a diagram or light entering the camera.It ensures that is incident ew — If ni oo wm — Hd The Mode of Action of the Lens Camera

Describe the mode of action of the lens camera camera (given as fraction of focal length F of lens) is also called F Number. a +The smaller the F – Number fora given focal length the larger the lens diameter +The lens with a larger diameter has greater light- gathering power or speed

• This for sucha lens the shuter allows light in the eamera fora short interval of time

‘The Magnification of the Lens Camera Determine the magnification of the lens camera Magnification of a lens camerais obiained as the ratio of the Image distance and the object distance But from the lens formula

Thus M=v/U

W=UF-W

(Wy t= (U-F/ Fuy!

Vv=FU/(U-F)

Example S

A lens camera is to be used to take a picture of a man 2m tall if the lens of the camera Has focal length of 10cm, ealewlate the minimum size ofthe film Frame required, given that the man is 20, Lm from the camera

Solution

Magnification is given by M=o (AH) Where

6s

Lot the size of the frame be h when the height of man is 2m. Then

nl (2/200)

The film frame should be at least 10mm square.

Simple Lens Camera

Construct a simple lens camera A simple lens camera Shutter Lens \, «& = ccd y Image Aperiie kp a

The Human Eye

‘The Structure of the Human Fye 6s Describe the structure of the human exe The eyeball approximately spherical in shape.The wall of this sphere consist of two layers, the ‘outer layer or sclera and the inner layer or choroid The front portion of the SCLERA FORMS A TRANSPARENT CURVED section called the camera The choroid layer is balance in order to prevent intemal reflection and also to protect the light sensitive parts of the eye

The aqueous and vitreous hum our are jelly ~ like substance that fills the spaces within the eyeball. The aqueous humour is the salt solution of refractive index n, 1.38,Vitrous burmour is @ watery , Jelly substance of refiactive index 1.34.Bchind the comea there is a colored diagram called the iis.

The iris has the central hole called the pupil. The iris contains muscles which control the size of the pupil. The size of the pupil decreased in the bright light and inereased in the dim light Behind the pupil and there is a crystalline fens held in position by suspensory ligaments that are attached to the choroid layer. Near the suspensory ligaments are the ciliary muscles. The function of the suspensor ligaments there are the cilliary muscles,

The function of cillary muscles is to control the thickness of the lens. The lens become thick when the ciliary muscles contract and thin when the eiliary muscles are relaxed. At the back of the eye there is a retina (This is the part of the eye which is sensitive to light).Image formed is inverted formed on the Retina ( This is the part of the eye which is sensitive to ight) Image formed is inverted formed on the retina by successive refraction of light at the comer, the

aqueous hurmour the crystalline lens and the Vitreous hurmour Electrical signals are then transmitted to the Brain through the topic nerve. Finally, the brain interprets these signals, a Cross section of Human Eye y > ,. 1 nvered image J fobjece opt \ 7s ect vee x 4 Optical eee

Ugamenes Seay

aay SS Muse Accommodation Power of the Human Eye Explain accommodation power of the human eye Accommodation is the process whereby the eye alters its focal length in order to form images of objects at different distances, (Thickening or Thinning of the lens causes a change in its focal length), The thickening or thinning of the crystalline lens is made possible by the action of the ciliary muscles.To view neare object t, ciliuary muscles contract, this makes the lens thicker.

In the relaxed state of ciliary muscles, the crystalline lens become thinner and enables the eye to see (view) distant objects. The farthest point which can be seen clearly is ealled the far point of the eye and the nearest point is called the near point of the eye.

The corresponding distance from these points to the eye are referred to as the maximum and least distance of district vision respectively A normal eye (ie without defects of vision) has a far Point at infinity and near point at a distance of 2Sem from the eye Structure of lens “view distant object” The Defects of the Human Eye Identify the defects of the human eve Myopia or near-sightedness

• This detect causes person to see near object clearly while distant objects are not seen

clearly

• The strength of the comea and the eye lens combination is too great even when muscles

of the eye are completely relaxed

• The focal length of the comea and the eye ~ lens combination is always less than the

distance to the retin.

• Images of distant object ere formed in front of the retina even when eye is totally relaxed

However, an object that is closer ean be brought into focus,

• In this situation the focal length of the cornea and the eye lens is so short that objects

closer than the conventional (near point of 25cm) can be brought into focus. ‘That's why this condition is ealled Short sightedness (near sightedness)

• Since the problem is that the strength of the eye — lens and the comea combination is too

great, the solution isto provide eye glasses (or contract lenses) with negative lens

• The negative lens weakens the strength of the comea and eye ~ lens just enough so that

the resulting foeal length when the eye muscles are relaxed matches the distance back to the retina so that distant images are now in focused +The eye glass lenses are negative lenses that means they are thinner in the middle than at the edges,

• Its easy to identify this kind of eye glass lenses since acting by themselves they do not

form a real image of an object at any distance. a a: a ———— Hyperopia or far-sightedness

• This defect causes a person to see distant objects only and short-distance objects are not

seen clearly. . In the person with this condition, the strength of the cornea and the eye-lens combination is too weak when the eye muscles are totally relaxed. So the image of a distant object is formed behind the retina

• The solution in the opposite of myopia. Vietims should wear positive eye lenses which

strengthen the corner and the eye lens just enough so that the resulting focal length when the eye is relaxed matches the distance to the back of the retina mates Astigmatism

• This oceurs when the focal length for the comea and the eye's lens for an object oriented

in some direction isnot the same as for another located in a perpendicular diteetion

• The eye can not bring the vertical and horizontal lines in a “+” symbol in sharp focus at

the same time, (The axis of differing focal length need not be exactly horizontal and vertical)

• The problem is that the comea of the eye lens is not symmetrical. The solution is to use

eye glasses whose lenses are not symmetrical in 2 complementary way

• The cylindrical lens may be combined with an additional positive or negative lenses

Decreased accommodation

• This condition typically oceurs in middle-aged people
• The oye muscles gradually weaken with age, so that the range or accommodation is

decreased.

• People with this condition cannot bring both near objects and far objects into focus,
• The weakening of the eye muscles often causes the focal length of the eye lens to

increase as well so that many people of middle age tend to become far sighted

• Since the problem is adequate accommodation, no single lens can eorreet it and people

with this problem usual needs bifocals,

• Bifocals are glasses with two different lens strengths, one for near and one for distant

objects.

• The usual arrangement is that the bottom half of the lens is the near strength and the top

half is the Far strength, ‘The Correction of the Defects of Human Eye Describe the correction of the defects of human eye Myopia is common name for impaired vision in wich a person sees near objects clearly while distant objects appear blurred. In such a defective eye, the image of a distantobject is formed in front of the retina and not at the retina itself, Consequently, a nearsighted person cannot focus clearly on an object farther away thanthe far point for the defective eye

This defect arises because the power of the cye is too great duc to the decrease in focal length of the erystalline lens. This may arise due to either n 1 excessive curvature of the comea, or 2 elongation of the eyeball.

Correetion:Thisdefectcan becorrectedey using aconcave (diverging) lens. A. concave lens of appropriate power or focal length i able to bring the image of the object back on the retina itsel Farsightedness, also called bypermetropia, common name for a defect in vision in which 2 person sees near objects with blurred vision, while distant objects appear in sharp focus. In this «ease, the image is formed behind the retina

This defect arises because either

• the focal length ofthe eyelens is too great, or
• the eyeball becomes too short, so that light rays from the nearby object, say at point N

‘cannot be brought to focus on the retina to give a distinet image. Correetion:This defect ean be corrected by using aconver(comverging) lensof appropriate focal length. When the object is at N’, the eye exerts its maximum power of accommodation.

Eyeglasses with converginglenses supply the additional focussing power required for forming the image on the retina ‘The Human Eye and the Lens Camera Compare the human eye and the lens camera ‘The camera 1 Theeye and the camera has @ havea convex Tens which form a real and inverted image of an object.

n 2 The eye and the camera are blackened inside to prevent internal reflection. Rays of Tight which are not received on the retina or eamera film are absorbed by the choroid layer of the eye ‘or the black surface inside the carmera

• The eye can regulate the amount of light that passes through the crystalline lens by using

pupil while in a camera the diaphragm regulates light 4 Inthe eye the image is formed in the retina while in the camera the image is formed on the photographie plate

• The eye can change the focal length of its lens by the contraction and relaxation of the

ciliary muscles. In this way the eye can focus objects at different distance. In a camera objects at different distance are focused on by moving the lens forwards and backwards. B