Question

The theory of the particle nature of light proposed by Isaac Newton and in order to support his theory, Newton had to explain the many behaviours of light using his theory. Discuss how Newton explained the following properties of light using this particle theory and its postulates: reflection, refraction, dispersion, rectilinear propagation, and diffraction, partial reflection-refraction.

Answer 1

Now days, after establishment of Quantum Mechanics it is cleared that wave has dual character and it has both natures existed simultaneously wave as well as particle. Here we will be discussed about particle nature of light, in early days Newton explained that light was made of very tiny particle and these particles called as "corpuscles" (little particles) which travel in a straight line with a finite velocity and called it corpuscular theory of light. On the basis of corpuscular theory Newton explained the reflection and interference (which is just like to reflection) by light.

Postulates of Newton’s theory of light:

· As discussed above, Newton proposed that a source of light emits tiny particles which are elastic, rigid and massless particles called corpuscles.

· These emitted particles travel in all direction with very high speed in transparent medium but path travelled by them are always straight i.e. these particles travels always in straight line.

· These tiny particles enter our eyes and produce the vision.

· These particles produce seven colours which are named as VIBGYOR.

· These light particles are repelled by a reflecting surface and attracted by transparent materials.

Theory of reflection as we are familiar that when we throw a solid body on a smooth surface then it come back to us here newton explained reflection like this, it was known that when light reflects from a smooth surface, the angle of incidence is equal to the angle of reflection.

Now in case of theory of refrection Newton thought that matter is made of particles (atoms). When a light(which is treated as particle) is within a medium, such as water or glass, it is surrounded on all sides by equal numbers of particles. Let us assume that, there are attractive force between the light particles and the matter particles. Then within a medium, these forces cancel each other out and there is no net force on the light particle. Then, according to Newton’s first law, the light particle will continue moving in a straight line since no net force acts on it. Near an interface the situation is different. Now there are more matter particles on one side than the other, and the light particle can experience a net force. It would experience a brief attractive force towards the medium with more matter particles. As the light particle moves into the water, it experiences a brief attractive force towards the water. This increases the vertical component of its velocity. Since it did not experience any net horizontal force, its horizontal velocity remains the same. This total force speeds the light particle up, and deflects its velocity towards the surface normal. Newton also explained the Snell’s law in case of refraction and finally got the result which is ratio of angle of incidence angle to the ratio of angle of refraction is equal to the ratio if velocity of medium to the velocity of air.    

In case of dispersion newton introduced the colour spectrum of light in which seven coloured are involved and named as VIBGYOR and now a days everybody familiar with VIBGYOR. Here Newton used prism to disperse the light which is also another kind of refraction in which white light passes through a prism and disperse into seven colour and no boundaries between them.   

Rectilinear Propagation: If we through a ball into space the n trajectory of ball is curved due to gravity to reduce the effect of gravity throw the ball with greater speed which reduced the curve and ideally, we can reach that speed where curve become straight line. Newton used this assumption for the corpuscular theory of light and said light travels in straight line. This property of light has great support against the wave theory because sound can be heard around the corner of obstruction but light cannot be seen behind the obstruction.

Diffraction can nit be explained by newton theory of light but in case of partial refraction and reflection is possible.

Conclusion:

Newton theory was not so good and which is not explain the wave nature of light due to which we were unaware of many things like diffraction and one more thing Newtons theory on refraction was not so satisfactory.