★ Solution :-

• Understanding the Figure ::

Here in this figure, the light ray passes from atmosphere into rain drops and then comes out with dispersed colours showing the beam of colours.

• Real Phenomenon : Either Total Internal Reflection or Internal Refraction ::

Actually the real process happening there is Internal Reflection not Total Internal Reflection. For Total Internal Refraction, the angle of the refracted ray from the atmosphere to the water drop, should be more than the critical angle. But here that isn't happening. Even in Total Internal Reflection after getting reflected back from the raindrop's surface, whole of the light get's reflected back in the direction it entered. Here the light ray is reflected in opposite direction.

You know why this change is occuring ?

Actually some of the light energy get's diffused making the speed of light change and thus the direction also changes. But in case of Total Internal Reflection, whole of light energy get's transmitted back.

In case of Internal Reflection only the left light energy get's transmitted. That's why it get's reflected only internally in the rain drop.

• Role of Total Internal Reflection ::

Actually the approximity of the diffusion of some light energy, is very less and the angle difference required for Total Internal Reflection is also less. When we go on studying the cases of Internal Reflection, we have different cases which aren't based on the concept of class 10. Thus for the convenience of the students, it is taken to be Total Internal Reflection by the negligence of the differences. But according to Ray Optics of class 12, it is made to understand the real phenomenon here.

In real, Total Internal Reflection isn't involved in Rainbow Formation.

• About the given diagram ::

The given diagram is exactly correct because the ray of light is clearly and correctly shown in this. Also there is proper marking of direction of the ray of light and also correct angle and phenomenons.

• Formation of Rainbow ::

Rainbow formation takes place due to three phenomenon that is Refraction, Dispersion and Internal Reflection. When the ray of light strikes the rain drop it refracts and thus enters in the rain drop. This causes change in it's speed due to difference in refractive index. Then it suffers dispersion which makes the bands of colours visible. This forms the colours of the rainbow. Then this band strikes the back surface of the raindrop and reflects internally in the raindrop facing internal reflection. And then we finally it refracts backs coming out of the raindrop as a band of seven colours which is visible to the different organisms.

Verified answer

Answer : )

The diagram is shown. The ray PQ enters the prism from the side AB, its angle of incidence is 0°. Thus it does not undergo any refraction. Its angle of refraction is also 0°. When this light ray reaches the base of the prism (BC), its angle of incidence (on the boundary BC) is 60° which is more than its critical angle (42°). Thus this incident light ray (QR) does not enter into the next medium, instead it undergoes total internal reflection. When this reflected light ray (RS) reaches the side AC, its angle of incidence is 0°. Again, it does not undergo any refraction, which causes the angle of emergence to become 0°.