The Speed of Light Slows Down in Diamonds

The brilliant sparkle of a diamond is the result of a significant traffic jam for light. This phenomenon is due to the diamond's high refractive index, a measure of how much a material can bend or slow down light. Diamond has an exceptionally high refractive index of 2.42, meaning that light travels 2.42 times faster in a vacuum than it does through the diamond. This dramatic deceleration causes the path of light to bend sharply as it enters the stone. This principle, known as refraction, was first described by Dutch mathematician Willebrord Snell in 1621, laying the groundwork for our understanding of optics.

This intense bending of light is precisely what makes a diamond so dazzling. A well-cut diamond is designed with specific angles and facets that act like a hall of mirrors. The light, now moving sluggishly and bent at a sharp angle, is trapped inside, bouncing from one internal facet to another in a phenomenon called total internal reflection. This process gives the diamond its brightness. Furthermore, as the light finally exits the stone, it doesn't leave as plain white light. The slowing effect also causes the light to split into its constituent spectral colors, creating the flashes of rainbow "fire" that make diamonds so captivating. It is this masterful manipulation of light's speed that creates the gem's iconic brilliance.