Question

Given below are two statements, one labelled as Assertion (A) and other labelled as Reason (R).

Assertion (A) : A diamond sparkles more than a glass imitation cut to the same shape.

Reason (R) : The refractive index of diamond is less than that of glass. In context of the above two statements, which one of the following is correct?

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true and R is not a correct explanation of A

(c) A is true but R is false

(d) A is false but R is true

Answer 1

(c) This is a phenomenon of total internal reflection of light, dependent on the$critical angle' of the incidence of light in a material medium at its bounding surface with air. The higher the refractive index of a transparent material the smaller is the critical angle and hence the larger is the range of angles of incidence for more light to be totally reflected. A diamond has a large refractive index and very small critical angle as against glass, which has a lower refractive index and large critical angle.

A skilled diamond cutter exploits the large range of angles of incidence in the diamond to cut multiple faces at suitable angles. Light entering diamond from different faces, suffers multiple total internal reflection and comes out of the diamond as intense beams from selected directions. Hence, a diamond shines brilliantly.