A diamond sparkles because its refractive index of 2.42 pulls the critical angle down to roughly 24°, so light that gets inside bounces around many times before it can escape. Water and glass have lower indices, so the threshold retreats to about 49° and 42°; as the angle of incidence sweeps from 0 to 90°, the transmitted ray fades and then vanishes exactly at that critical angle, leaving only total internal reflection inside the medium. The same law of refraction governs a convex lens, where the object’s position relative to the focal point decides whether the image is real or virtual, and governs the eye, where focusing power decides whether the image lands on the retina or falls short of it or beyond.
A diamond sparkles because its refractive index of 2.42 pulls the critical angle down to roughly 24°, so light that gets inside bounces around many times before it can escape. Water and glass have lower indices, so the threshold retreats to about 49° and 42°; as the angle of incidence sweeps from 0 to 90°, the transmitted ray fades and then vanishes exactly at that critical angle, leaving only total internal reflection inside the medium. The same law of refraction governs a convex lens, where the object’s position relative to the focal point decides whether the image is real or virtual, and governs the eye, where focusing power decides whether the image lands on the retina or falls short of it or beyond.
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