Question

n21=_______________________​

Answer 1

Answer:

1 n 2= refractive index of medium 2 with respect to medium 1.

What does the notation n21 means can be found from the text/context.

It could mean refractive index of medium 2 wrt medium 1 or the other way round. Read carefully the whole text and the notation would become clear.

Explanation:

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Answer 2

(´∩｡• ᵕ •｡∩`) See this one also choose