Question

(i) Explain what happens to the ray of light in the above three diagrams.
${}_2n_1 = {{v_1 } \over {v_2 }}$
(ii) Explain the meaning of the above equation.

Answer 1

Refraction of light through glass slab helps in explaining the given diagram which has three cases of refraction (i) rarer to denser (ii) denser to rarer (iii) normal incidence. The given equation is for index of refraction.

Complete answer:
(i) The diagrams given depict the refraction of light through a glass slab. The denser medium can be treated as a glass slab. The rarer medium can be air/vacuum. The angle that incident ray makes with the normal is incidence angle (i), the angle that refracted ray makes is angle of refraction (r). Discussing the three cases in the following manner:
(1) As the light passes from rarer to denser medium, the light ray bends towards the normal. Therefore, the angle of incidence will be greater than the angle of refraction.
(2) As the light passes from denser to rarer medium, the light ray bends away from the normal. Therefore, the angle of incidence will be less than the angle of refraction this time.
(3) In the case of normal incidence, the light passes undeviated. The angle of incidence is zero degrees i.e., incident light is parallel to the normal. The angle of refraction is also zero degrees.
(ii) Now, the expression given is for the index of refraction. If the light passes from medium 2 to medium 1, the index of refraction can be expressed as:
${}_2n_1 = {{v_1 } \over {v_2 }}$
Here, $v_1$
is the speed of light in medium 1 and $v_2$
is the speed of light in medium 2.

Note:
There is a separate name for the angle r in case 2 (or diagram 2). The light happens to be emerging out of the denser medium, back into the rarer medium, hence the name given to that angle is angle of emergence.