Question
Why does diamonds sparkle for sometimes in the dark?
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Answer
Critical angle for diamond is $24^{\circ}$, for the ray of light travelling from diamond to air. Moreover, the diamonds are usually cut at very sharp angles, such that once a ray of bight enters into it, it surfers a series of total invernal reflections. When the angle of incidence at any face is less than $24^{\circ}$, the light emerges out from it and as a whole the diamond appears to be bright. Thus, the ray on entering a cut diamond, gets trapped within it for some time and causes it to sparkle in the dark.
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