Historical development of theories of light:

Isaac Newton proposed corpuscalar theory of light in 1672. Christian Huygens proposed wave theory of light in 1678. James Clerk Maxwell proposed electromagnetic wave theory of light in 1865. Albert Einstein proposed photon theory of light in 1905. Louis de Broglie proposed wave particle duality theory of light in 1924.

Newton’s corpuscalar theory:

According to Newton’s corpuscalar theory:

a) Every source of light emits large number of tiny particles known as ‘corpuscles’ in a medium surrounding the source.

b) These corpuscles are perfectly elastic, rigid, and weightless.

c) The corpuscles travel in a straight line with very high speeds, which are different in different media.

d) Observer gets sensation of light when the corpuscles fall on the retina.

e) Different colors of light are due to different sizes of corpuscles.

f) This theory successfully explains reflection, refraction, and inverse square law.

Drawbacks of Newton’s corpuscalar theory:

Newton’s corpuscalar theory has following drawbacks:

a) It cannot explain partial reflection and refraction at the surface of a transparent medium.

b) It cannot explain phenomena like interference, diffraction, and polarization.

c) The theory predicts that speed of light in denser medium is more than the speed of light in a rarer medium. However, experimental findings are against this prediction.

d) When particles are emitted from the source of light then mass of the source of light must decrease. However, experimental findings are against this prediction.

Characteristics of Huygen’s wave theory of light:

Characteristics of Huygen’s wave theory of light are as follows:

(a) Light travels in the form of longitudinal elastic waves which travel with uniform velocity in homogenous medium.

(b) Different colors of light are due to different wavelengths of waves.

(c) When light waves enter eyes of observer, he or she gets sensation of light.

(d) A material medium is necessary for propagation of longitudinal elastic waves. Huygens suggested that space is filled with hypothetical medium called ‘ether.’

Successes of Huygen’s wave theory of light:

Successes of Huygen’s wave theory of light are as follows:

(a) It could explain the laws of reflection, refraction, interference, diffraction, etc.

(b) It predicted that speed of light in optically denser medium is less than that in an optically rarer medium in agreement with experimental results.

Drawbacks of Huygen’s wave theory of light:

Drawbacks of Huygen’s wave theory of light are as follows:

(a) It could not explain rectiliner propagation of light.

(b) It could not explain phenomenon of polarization of light and phenomenon like Compton effect.

(c) Michelson Morley experiment ruled out the existence of ether.

Wavefront:

A wavefront is a surface passing through all points in the same phase. Alternatively, a wavefront is defined as surface of constant phase.

Wave normal:

A normal to the wavefront at any point, is called the wave normal at that point.

Huygen’s principle:

Every point on a wavefront acts as a secondary source of light sending out secondary wavelets in all directions. The tangent to all these secondary wavelets at any instant gives the new position of the wavefront at that instant.

Discovery of polarization:

In 1808, Malus discovered the phenomenon of polarization by reflection. When a beam of unpolarized monochromatic light is incident on a plane glass plate, part of the light is reflected while the rest is transmitted. The reflected light is partially polarized. At a certain angle of incidence, the reflected ray of light is completely plane polarized.

Polarization of light:

The phenomenon in which the transverse vibrations of light are restricted to a definite plane or direction is called polarization of light. Polarization proved that light consists of transverse waves.

Unpolarized light:

The ray of light which consists of vibrations of light are in all possible directions is called unpolarized light.

Plane polarized light:

When the vibrations of light are confined in one plane, the light is called plane polarized light.

Plane of vibration:

The plane in which the vibrations of polarized light take place is called as plane of vibrations. In literature there seems to be some confusion regarding the definition of this term.

Plane of polarization:

The plane perpendicular to the plane of vibration is called plane of polarization. There are no vibrations in the plane of polarization. In literature there seems to be some confusion regarding the definition of this term.

Transmission Axis:

Transmission axis of polariser is the axis through which the component of electric field (of the incident electromagnetic wave) parallel to this axis passes through polariser. Also, the component of electric field (of the incident electromagnetic wave) that is perpendicular to this axis is simply blocked.

Intensity Law:

If ordinary light of intensity I₀ is incident on polarizer then the intensity of plane polarized light coming out through polarizer, I, is given by:

I = (1/2) x I₀

where I = intensity of light coming out through polarizer, I₀ = intensity of ordinary light that is incident on polarizer.

Malus Law:

If a plane polarized light of intensity I₀ falls on analyzer then the intensity of light coming out through analyzer is given by:

I = I₀ x cos²θ

where I = intensity of light coming out through analyzer, and θ = angle between transmission axes of analyzer and polarizer (the polarizer which polarized the incident light on analyzer).

Brewster’s law:

The tangent of the polarizing angle is equal to the refractive index of the refracting medium at which partial reflection takes place. Alternatively, in terms of an expression:

μ = tan i

where, μ = refractive index of the refracting medium, i = polarizing angle.

Polarizing angle:

Polarizing angle is the angle at which ordinary light should incident on the non-conducting surface so that reflected light should be plane polarized to maximum extent.

Polarisation and electromagnetic waves:

Polarisation can be explained with the help of electromagnetic wave theory of light. In electromagnetic wave theory of light, waves are transverse. If a ray of light is travelling along X-axis then it consists of electric vibrations in xy-plane and magnetic vibrations in xz-plane (i.e., electric and magnetic vibrations are perpendicular to each other). Only electric vibrations are responsible for polarisation.

Dichroism:

The property by which some doubly refracting crystals absorb the ordinary rays (O- rays) completely and extraordinary rays whose direction is parallel to the optic axis while passing through the crystal, is called dichroism.

Dichroic crystal:

The crystals possessing dichroism property is called dichroic crystal. Example: tourmaline crystal.

Polaroid:

It is a large sheet of synthetic material packed with tiny crystals of a dichroic substance oriented parallel to one another, so that it transmits light only in one direction of the vibration (i.e., electric vector). When unpolarized light is allowed to incident on polaroid then plane polarized light emerges through it.

Uses of polaroid:

Uses of polaroid are as follows:

(a) In motor car head lights to remove headlight glare.

(b) In three dimensional movie cameras.

(c) To produce and analyze polarized light.

(d) It is used as filter in photographic cameras.

(e) In window of aeroplanes to control the amount of light coming in aeroplane.

(f) In polarizing sunglasses (goggles) to protect the eyes from glare of sunlight.

(g) To improve color contrast in old oil paintings.

Double refraction:

Double refraction means birefringence. See "birefringence" given below for its definition.

Birefringence:

Birefringence is also called "double refraction."" Birefringence is the process in which a ray of ordinary light - when it passes through a birefringent crystal - splits into two plane polarized components. These two components travel with different velocities and are poarized at right angle to each other.

Birefringent crystal:

The crystal that possesses and exhibits the birefringence is called birefringent crystal. Birefringent crystals are widely used in manufacturing polarizing prisms and quarter wave plates.

Extraordinary ray (E-ray):

When a ray light passes through birefringent crystal, it splits into two rays, namely, ordinary ray (O-ray) and extraordinary ray (E-ray). The ordinary ray passes through crystal without any deviation. The extraordinary ray, however, refracts and travels by making some angle with ordinary ray.

Polarizer:

Polarizer is a device that converts ordinary light into plane polarized light.

Analyzer:

Analyzer is a device that detects whether light incident on it is ordinary light or plane polarized light. When a beam of light is incident on analyzer, slowly rotate the analyzer. If intensity of light doesn't change then incident light is oridnary light. If intensity of light varies from maximum to minimum and then again from minimum to maximum then incident light is plane polarized light.

Blue shift:

Let source of light and observer are approaching each other, and frequency of light be f. Then observer finds that frequency of light is f + df. Thus observer finds that frequency of light is more than f. This phenomenon is known as blue shift because frequency of light is shifted to blue color (higher frequency side) in spectrum.

Red shift:

Let source of light and observer are receding from each other, and frequency of light be f. Then observer finds that frequency of light is f – df. Thus observer finds that frequency of light is less than f. This phenomenon is known as red shift because frequency of light is shifted to red color (lower frequency side) in spectrum.