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Question
What are Secondary sources? State Huygens’ Principle.
Solution
Secondary sources are those sources that do not produce light of their own but receive light from some other source and either reflect or scatter it around.
Examples: the moon, the planets, objects.
Statement of Huygens’ principle: Each point on a wavefront acts as a secondary source of light emitting secondary light waves called wavelets in all directions which travel with the speed of light in the medium. The new wavefront can be obtained by taking the envelope of these secondary wavelets travelling in the forward direction and is thus, the envelope of the secondary wavelets in forwarding direction. The wavelets travelling in the backward direction are ineffective.
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Answer in brief:
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