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(B) The refractive index $n$ is defined as the ratio of the speed of light in vacuum (or air) to the speed of light in the medium,$n = \frac{c}{v}$.Gi

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$\frac{i}{5}$

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(B) The refractive index $n$ is defined as the ratio of the speed of light in vacuum (or air) to the speed of light in the medium,$n = \frac{c}{v}$.Given that the velocity is reduced by $20 \%$,the new velocity $v = c - 0.20c = 0.80c = \frac{4}{5}c$.Thus,the refractive index $n = \frac{c}{v} = \frac{c}{0.8c} = \frac{1}{0.8} = 1.25$ or $\frac{5}{4}$.According to Snell's Law for small angles,$n_1 \sin i = n_2 \sin r$. Since the incident medium is air $(n_1 = 1)$,we have $i = n r$,so $r = \frac{i}{n} = \frac{i}{1.25} = 0.8i = \frac{4i}{5}$.The angle of deviation $\delta$ is given by $\delta = i - r$.Substituting the value of $r$,we get $\delta = i - 0.8i = 0.2i = \frac{i}{5}$.

$A$ ray of light travelling through a rarer medium is incident at a very small angle $i$ on a glass slab,and after refraction,its velocity is reduced by $20 \%$. The angle of deviation is:

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