Question

Brilliance of diamond is due to
(A). Shape
(B). Cutting
(C). Reflection
(D). Total internal reflection

Answer 1

- Hint: Diamond when seen at certain angles sparkles brilliantly, as all the light entering emerges through those directions. The idea of total internal reflection needs to be considered. Diamonds are processed to maximize the effect of reflection.

Complete step-by-step solution -
i) When light passes from the denser medium to a rarer medium, it bends away from the normal. As the angle of incidence is increased, the bend increases and at some angle the refracted light is so bent that it grazes along the interface. This angle is defined as a critical angle.
ii) Beyond this angle of incidence, all of the light hitting the interface gets reflected back. This phenomenon is called total internal reflection.
iii) The sine of critical angle is inversely proportional to the refractive index of the denser medium.
iv) Diamond has a refractive index of 2.42. Due to this high refractive index, the critical angle for diamond is 24°. As a result of this low critical angle, more than 70% of all the light hitting at different angles has the possibility of getting totally internally reflected causing multiple reflections inside the diamond.
v) However, this alone isn’t responsible for the sparkle of diamond as can be seen in an unprocessed diamond. To maximise the effect, the diamond is processed and is finely cut in some definite shapes. As a result, all of the light entering the diamond goes through several internal reflections and exit only in some particular directions. When the diamond is viewed from these angles, it sparkles brilliantly.
vi) Hence, the main reason behind the brilliance of diamond is the total internal reflection of light, while the cut and shape boost the process.

Therefore, the correct option is D.

Note: The appropriate answer would be all the given options, since the total internal reflection along with the cut and shape of the diamond results in the brilliance of it. So, students sometimes get confused with this.