¶¶ A light ray striking at one face of a triangular prism gets refracted twice & emerges out from the other surface. The angle between the emergent and the incident ray is called the angle of deviation.

¶¶ so "D" is correct

The angle between the incident ray and the emergent ray of a prism is called the angle of deviation. Let's understand why this is the correct answer in detail.

- In a prism, light undergoes refraction at two surfaces. When a ray of light enters the prism, it bends towards the base of the prism at the first surface (incident ray). The ray then undergoes another refraction at the second surface and emerges from the prism (emergent ray).

- The angle between the incident ray and the emergent ray is known as the angle of deviation. This angle measures how much the light is deviated or bent from its original path as it passes through the prism.

- The angle of incidence refers to the angle between the incident ray and the normal (a line perpendicular to the surface) at the point of incidence. It is not specifically related to the prism itself, but rather describes the angle at which light strikes any surface.

- The angle of emergence refers to the angle between the emergent ray and the normal at the point of emergence. Again, this angle is not specific to the prism but rather describes the angle at which light emerges from any surface.

- The angle of refraction refers to the angle between the refracted ray and the normal at the point of refraction. It is related to the bending of light as it passes from one medium to another, but it does not specifically describe the angle between the incident and emergent rays of a prism.

- The angle of deviation, on the other hand, is unique to the prism and represents the angle between the incident and emergent rays. It determines how much the light is bent or deviated from its original path. By measuring the angle of deviation, we can determine the refractive index of the prism material and study the dispersion of light.

In conclusion, the correct answer is option 'D' - angle of deviation. This angle specifically refers to the angle between the incident ray and the emergent ray of a prism.