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