Total internal reflection is the main reason diamonds sparkle so brilliantly. A diamond has a very high refractive index, which means it bends light significantly when light enters from the air.
When light enters a diamond, it slows down and bends due to the high refractive index. Inside the diamond, the light hits the internal surfaces at angles greater than the critical angle, causing total internal reflection. Instead of passing out of the diamond, the light reflects multiple times inside.
This repeated reflection keeps the light trapped inside until it finally exits through the top, often after being split into different colors through dispersion. The many internal reflections make the light emerge in various directions, creating the intense sparkle, or “fire,” that diamonds are known for.
The diamond’s cut enhances this effect. A well-cut diamond ensures that light entering it is internally reflected in such a way that it exits through the top, directly into the viewer’s eyes. Poorly cut diamonds allow light to escape from the sides or bottom, reducing the sparkle.
Thus, total internal reflection in combination with the diamond’s shape and high refractive index causes light to bounce around inside and exit in dazzling bursts.