Ophthalmic Lenses Lecture 1

OPHTHALMIC LENSES

Ophthalmic Lenses Lecture 1

An understanding of lens optics begins with basic study of the action of a single ray of light and how it is affected when passing into or through a transparent optical surface.

INTRODUCTION…

To understand the way light behaves for lenses we need to look at the nature of light itself.

• When light travel it behaves in two ways,
• Like a wave generated by dropping a rock into a pond.
• As a particle or photon.

THEORY OF LIGHT

For our purposes, we can best understand light as a wave.

Deflining light wave

Note: The greater the amplitude the greater the intensity of the light.

As wavelength changes, so does the perceived color of the light.

The wavelength of light that are visible to the human eye vary in length from 380 to 760nm.

Color of high or largest wavelength is more seen than with the short wavelength.

Red (620-760), orange (590-620), yellow (560-590), green (490-560), blue (450-490) and violet (380-450).

• These are only a small part of the electromagnetic spectrum.

The visible spectrum

Reflection

Is the bouncing back of light rays when interrupted on its path at an angle.

Angle of incidence(i) ; The angle at which the light strikes the surface. Measured from a line perpendicular to the reflecting surface at the point of reflection known as the normal to the surface.

Properties of light

Angle of reflection; Angle at which the ray is reflected.

ie i = r

2. Speed of light and refractive index

• Light is able to travel faster through some material than through others.
• Travels more faster in a vacuum , because there is nothing to resist than other medias.

Speed of light in a vacuum is 3×10*8m/s .

The medium of less resistance is called rarer and the medium of more resistance is called denser medium.

The amount of resistance to the speed of light that slows it down is called Refractive Index.

Note; The more the material slows the passage of light , the higher is its refractive index.

Refractive index =speed of light in vacuum/speed of light in a new substance.

The refractive index is always greater than one , the speed in the denominator is always slower and smaller number.

3. Refraction.

Happens when light strikes a new transparent medium straight , the light slows down , but continues on the same direction.

When light strikes a new substance , the change in speed in the new media causes the light to change direction.

Ophthalmic Lenses Lecture 1

Note ; when light travel from a rarer media to denser media the direction of the refraction is towards the normal to the surface and vice versa.

Snell‘s Law.

If the refractive index of both media through which light is travelling are known, the angle of refraction for a given angle of incidence is predictable.

• Mathematically,

nsini=n’sinr

Given, n =1 (refractive index of air)

• n’= 1.523 (refractive index of glass)
• I = 30degrees
• r= ?
• d= ?

Example,

4. Dispersion (white light split into its constituent colors).

• 5. Diffraction
• 6. Interference
• 7. polarization.
• 8. Dual nature (both wave and particles)
• 9. Absorption and transmission.

Other properties of light.

Introduction to ophthalmic lenses.