Huygens wave theory

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Huygens wave theory

This theory tells us as to how new waves are formed & how they propagate.
  1. Light travel in form of longitudinal waves, with uniform velocity in homogeneous medium.
  2. Different colors are due to different wavelengths.
  3. When wave enter our eye & gives energy we have sensation of light.
  4. The waves are formed in a hypothetical medium "ether" which is present everywhere in universe
Success –
  1. It could explain Interference, diffraction, Reflection & Refraction.   
  2. It predicts that in denser medium its wave length will be decreased hence velocity will decrease.
Experiments proved it. (Supported it)

Failure-
  1. It could not explain polarization, Compton effect, photo electric effect. And light is transverse.
  2. Existence of ether could not be proved as there are no traces of drag by Michelson Morley experiment.
  3. Energy of wave does not depend on its amplitude but depends on frequency.
  4. Color does not depend on wavelength. According to Maxwell; color depends upon frequency.

Huygens Principle of propagation of wave front

1) Every point on a given wave front called primary wave front act as a source of new disturbance called secondary wavelets which travel with speed of light in all direction
2) A surface touching these secondary wavelets tangentially in the forward direction gives the direction of secondary wave front
He made an adhoc assumption that amplitude of secondary wavelets is maximum in the forward direction & zero in backward direction.

Super position principle

According to super position principle, when two or more wave motions travelling through a medium super impose one another, a new wave is formed in which resultant displacement (Y) at any instant is equal to the vector sum of the displacements of individual waves (Y1+Y2) at the instant.
                                   i.e.   Y = Y1 + Y2 + Y3……+Yn
for example: - when crest of one wave falls on crest of the other, the amplitude of the resultant wave is sum of the amplitudes of two waves, as shown in fig (a) when crest of one wave falls on trough of the other, the amplitude of resultant wave is difference of the amplitudes of the two waves, as shown in fig (b).

Interference of light 


 At some points, where the crest of one wave falls on crest of others, resultant amplitude is maximum. Hence the intensity of light is maximum. At certain other points, crest of one wave falls on trough of the other resultant amplitude becomes minimum (zero) & hence intensity of light is minimum (zero). This kind of modification in energy distribution is called interference.

 click here to know about Dispersion, Diffraction & Polarization

 At the points, where the resultant intensity of light is maximum interference is said to be constructive. At the points, where the resultant intensity of light is minimum the interference is said to be destructive.