CBSE Class 12-science Answered

Diamonds sparkle is based on the total internal reflection. Higher the refractive index of a transparent material and smaller the critical angle, the larger is the range of angles of incidence for more light to be totally reflected.

A diamond has a large refractive index(2.42) and very small critical angle(24°) as against glass which has a lower refractive index(1.5) and large critical angle(42°).

As any angle larger than critical angle is totally reflected internally, the smaller the critical angle the easier it is to get internal reflections which cause the sparkle. As c is inversely proportional to n, higher the value of n, lower the c.

Hence in diamond, light entering the diamond at an angle of incidence between 24 ° and 90° gets totally reflected several times inside and emerges out ,which produces a sparkling effect in diamond. For glass the critical angle being higher the range of angle of incidence for more light to get reflected is less compared to diamond.