Question

A piece of diamond sparkles when viewed from certain directions

Answer 1

Refraction takes place , light from the air enters into the diamond, that is from a rarer to denser medium.

So, diamond shines bright.

Hope this helps you mate...

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Answer 2

Refraction occurs. Light in the air passes through the diamond ie., Rarer medium to denser medium.

Diamonds has a greater refractive index of 2.42. And sparkles when viewed from several directions

Hope u got ur answer.