When a ray of light is incident normally on one refracting surface

1. Refraction:

(i) The phenomenon of bending of light entering from one medium to another is called refraction of light.

(ii) When a ray of light travels from a rarer medium to a denser medium, it bends towards the normal.

(iii) When a ray of light travels from a denser medium to a rarer medium, it bends away from the normal.

(iv) There are two laws of refraction.

(a) First Law: The incident ray, the refracted ray and the normal at the point of incidence, all lie on the same plane.

(b) Second Law: The ratio of sine of angle of incidence to the sine of angle of refraction is constant for a given pair of media. This is also called Snell’s law.

2. Refractive Index: 

(i) Refractive index (n) of a medium is the ratio of speed of light in vacuum (c) to the speed of light in the medium (v) and is given by n=cv.

(ii) Refractive index of one medium w.r.t to another medium is given by, n21=Speed of light in medium 1Speed of light in medium 2.

3. Reversibility of light:

When a light ray, after suffering many reflections and refractions, has its final path reversed and travels back along its initial path is called principle of reversibility of light.

4. Lateral displacement: 

The perpendicular distance of separation between the incident ray and the emergent ray is called lateral displacement.

5. The apparent depth: 

The shift by which an object appears to be raised when it is immersed in water is called the apparent depth. Refractive Index=Real depthApparent depth.

6. Advances sunrise and delayed sunset:

Advanced sunrise and delayed sunset, the twinkling of stars and shimmering effect around a campfire are some consequences of refraction. 

7. Prism:

(i) A transparent refracting medium bounded by five plane surfaces inclined at some angles is known as a prism.

(ii) The angle between two refracting surfaces is called angle of prism (A) and the angle between the incident ray and the emergent ray is called the angle of deviation (D).

(iii) The refractive index of the prism, n=sinDmin+A2sinA2

8. Total internal reflection: 

(i) Critical angle for a pair of media is defined as the angle of incidence in denser medium for which the angle of refraction becomes 90°.

(ii) When a ray of light is travelling from a denser medium to rarer medium, is incident at the interface of the two media at an angle greater than the critical angle, the ray is totally reflected back to the denser medium. This phenomenon is called total internal reflection.

When a ray of light is incident normally on one refracting surface of an equilateral prism of refractive index 1.5, the emerging ray

B) is deviated by

Correct answer is D.

Critical angle for the material of prism

since angle of incidence at surface