Question

Total internal reflection takes place, when light travels from
A. Water to glass
B. Glass to diamond
C. Water to air
D. Air to mercury

Answer 1

Hint: Total internal reflection is an optical phenomena in which waves, such as light, are entirely reflected when they reach the border between two mediums, such as air and water, under specific conditions. When waves moving in one media collide with the interface of another medium with a greater wave speed (lower refractive index), they are totally reflected back into the first ("internal") medium, rather than refracted into the second ("external") medium.

Complete step by step solution:
A beam of light travels from a watery medium to one of air. The lightray will be refracted at the point where the two mediums meet. The refracted light beam bends away from the normal as it passes from a medium with a higher refractive index to one with a lower refractive index. The incident beam of light is refracted in such a way that it travels over the water's surface at a certain angle of incidence. The crucial angle is the angle of incidence at which something happens. The refraction angle is 90 degrees here. The incident ray is reflected back to the medium when the angle of incidence is larger than the critical angle. This behaviour is known as complete internal reflection.
Refraction is generally accompanied by partial reflection. When waves are refracted from a medium of lower propagation speed (higher refractive index) to a medium of higher speed—e.g., from water to air—the angle of refraction (between the outgoing ray and the surface normal) is greater than the angle of incidence (between the incoming ray and the normal)
Therefore the correct answer is option C.

Note:
The angle of refraction approaches 90 degrees as the angle of incidence approaches a specific threshold, known as the critical angle, at which the refracted ray becomes parallel to the border surface. The requirements of refraction can no longer be met when the angle of incidence approaches the critical angle, thus there is no refracted beam and the partial reflection becomes total. The critical angle for visible light incidence from water to air is around 49 degrees, and for incidence from ordinary glass to air is about 42 degrees.