## Sunday, 6 November 2016

### Nature of light : theories of light

Different theories of light:

i) Newton's corpuscular theory of light

ii) Huygen's wave theory of light

iii) Maxwell's electromagnetic theory of light

iv) Planck's Quantum theory of light

v) De-Broglie's dual theory of light

Newton's corpuscular theory of light:

characteristics:

a) Extremely minute very light and elastic particles are being constantly emitted by all luminous bodies in all directions.

b) These corpuscles travel with the speed of light

c) When these corpuscles strike the retina of our eye then they produce the sensation of vision.

d) The velocity of these corpuscles in vacuum is 3 x m/s.

e) The different colours of light are due to different size of these corpuscles.

f) The rest mass of these corpuscles is zero.

g) The velocity of these corpuscles in an isotropic medium is same in all directions but it changes with the change of medium.

h) These corpuscles travel in straight lines

i) These corpuscles are invisible.

The phenomenon explained by this theory

a) Reflection and refraction of light

b) Rectilinear propagation of light

c) Existence of energy in light.

but this theory could not explain ......Interference, diffraction, polarisation, double refraction and total internal reflection. Photoelectric effect and compton effect.

defect: Velocity of light being greater in rarer medium than that in a denser medium.

WAVE THEORY OF LIGHT:

-This theory was enunciated by Huygen in a hypothetical medium known as ether. Ether is that imaginary medium which prevails in all space, is isotropic, perfectly elastic and massless.

-The different colours of light are due to different wavelengths of these waves.

-The velocity of light in a medium is constant but changes with change of medium.

-This theory is valid for all types of waves.

Theory of secondary wavelets

characteristics of this theory.....

*The locus of all ether particles vibrating in same phase is known as wavefront.

*When light travels in a medium then the particles of medium start vibrating and consequently a disturbance is created in the medium.

*Every point on the wavefront becomes the source of secondary wavvelets.  It emits secondary wavelets in all directions which travel with the speed of light.

*The tangent plane to these secondary wavelets represents the new position of wavefront.

The phenomena explained by this theory.....

a) Reflection, refraction, interference, diffraction, polarisation and double refraction.

b) Rectilinear propagation of light.

c) Velocity of light in rarer medium being greater than that in denser medium.

d) Photoelectric effect, compton effect and raman effect.

e) Backward propagation of light ELECTROMAGNETIC THEORY....

This theory was enunciated by Maxwell.

characteristics:

-The waves produced by accelerated charges or oscillating circuits or high frequency A.C. in conductors are known as electromagnetic waves.

-In these waves the electric vector (Ē) and magnetic field vector (B) and the direction of propagation of waves are mutually perpendicular to each other.

-The nature of these waves is transverse.

-these waves can be polarised.

-In these waves electric and magnetic fields vibrate in the same phase i.e. these reach the extreme position and mean position simultaneously.

-these waves propagate in vacuum also.

-the velocity in vacuum is equal to that of light

-their velocity in vacuum is equal to that of light.

QUANTUM THEORY OF LIGHT:

This theory was enunciated by Max Planck.

characteristics :

-Light travels in the form of minute packets of energy which are known as quanta or photons.

Photons are emitted discretely and not continuously.

-The energy of photons is given by E=nhv (v-->frequency)

where n=1,2,3,...

n is the integer and it represents the number of photons.

-The velocity of photons is equal to that of light

-Their rest mass is zero.

-In this theory, the energy of all the photons is same whereas in Newton's corpuscles theory, corpuscles may possess any energy.

- It is stable particle.

-Being uncharged, these are not affected by electric and magnetic fields.