Class 12 Physics Wave Optics Light: A particle or a wave

Light: A particle or a wave?

  • Around 1800 and 1900, some of the scientists considered that light is a particle and according to some it was a wave.
  • According to Newton’s theory:-
    • Light is of particle nature. This means light is made up of small particles and those particles are known as corpuscles.
    • This theory was also known as corpuscular theory.
    • He proved the following phenomena by using the theory of mechanics :-


  • Consider the case of elastic collisions likein carom board.When we hit the striker, it strikes the boundary and bounces back.
  • When the striker hits the boundary of carom board it is same as an incident ray. When it bounces back it is the reflected ray.
  • Therefore in case of elastic collisions angle of incidence (i) is same as angle of reflection(r).
  • He was able to prove that the phenomenon of reflection of light by considering light of particle nature.


  • When light moves from a rarer medium to the denser medium it will bend towards the normal and when it moves from denser to rarer it bends away from the normal.
  • According to Newton when the corpuscles strike the boundary, the particles of the denser medium tend to attract corpuscles.
  • When the particles are deep inside the medium they will be attracted by all the other particles present in the medium.
  • The net force on the corpuscles is when they are fully inside the denser medium.
  • According to Newton’s first law of motion, if the net force is 0, then the particle will continue to move in a straight line.
  • Therefore as the net force on the corpuscles entering the denser medium is 0, they keep moving along a straight path.
  • As there are attractive forces in one direction the corpuscles get accelerated. As a result there is increase in velocity. The particles slightly bend towards the normal.

Rectilinear motion of light:-

  • Newton assumed there are 2 rooms side by side and are separated by a wall.
  • If the window of first room opens, the light will enter the room.
  • Light won’t enter the second room as both the rooms are separated by a wall.
  • According to him as light travels in straight line and whenever there is obstruction in between the light won’t pass through the wall.
  • This shows that light always travels in straight line.

  • The particle theory was accepted according to which light is made up of huge number of small - small particles known as corpuscles.

Shortcomings of corpuscular theory

  1. This theory could not explain the phenomenon of refraction properly.
  2. When the corpuscles enter from rarer medium to denser medium,they get accelerated i.e. their velocity increases.
  3. This shows the velocity of the light particles in the denser medium is greater than the velocity of the light particles in the rarer medium.
  4. This means velocity of light in denser medium is more than rarer medium.
  5. Newton was not able to explain why the velocity of light is more in denser than in rarer medium.

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