Question

White light is incident from under water on the water-air interface. If the angle of incidence is slowly increased from zero, the emergent beam coming out into the air will turn from
A. white to violet
B. white to red
C. white to black
D. None of the above

Answer 1

Total internal reflection is the phenomenon of bouncing back of light in the same medium when the light strikes the boundary of a rarer medium. When the angle of incidence is increased, angle of refraction also increases. But when the angle of incident exceeds the critical angle, light gets totally reflected back to the same medium i.e. no refraction of light. Use this basic information to answer this question.

Complete step-by-step answer:
White light is composed of 7 colors namely Violet (V), Indigo (I), Blue (B), Green (G), Yellow, (Y), Orange (O) and Red (R) i.e. VIBGYOR. When we optically combine these colors, we get white light. When white light is incident under water on the water-air interface and the angle of incidence is increased slowly from zero, each color from the colors mentioned above gets totally internally reflected one-by-one. Due to total internal reflection no light will pass.
Hence, if the angle of incidence is slowly increased from zero, the emergent beam coming out into the air will turn from white to black.
So, the correct answer is option C i.e. white to black.

So, the correct answer is “Option C”.

Note: Students must remember there are few requirements for total internal reflection which are as follows:
1. The incident light should be travelling in a more optically denser medium and approaching a medium which is optically less dense. Water is optically denser as compared to air.
2. The angle of incidence should be greater than the critical angle.