Question

2. Refractive index of glass relative to water is 9/8. What is the refractive index of water relative glass? (AS)
(Ans: 8/9)

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Answer 1

$n_{21} = \frac{speed \: of \: light \: in \: first \: medium}{speed \: of \: light \: in \: second \: medium}$

Given:-

$\frac{9}{8} = \frac{speed \: in \: first}{speed \: in \: second} = n_{12}$

and,

$n_{12} = \frac{1}{n_{21}}$

$n_{12} = \frac{8}{9}$

so,

refractive index water relative to glass is 8/9