Q. Assertion :Diamonds are known for their spectacular brilliance, but diamonds found in nature rarely exhibit the brilliance
Reason: By cutting the diamond suitably, multiple total internal reflections can be made to occur.

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Solution:

The critical angle for diamond-air interface is very small, therefore once light enters a diamond, it is very likely to undergo total internal reflection inside it. It is the technical skill of a diamond cutter which makes diamonds to sparkle so brilliantly.