Explanation:
B: Given, $\mu_{\mathrm{c}}=1.68, \mu_{\mathrm{o}}=1.2$
We know, refractive index of crystal w.r.t. oil,
${ }_{\mathrm{o}} \mu_{\mathrm{c}}=\frac{\mu_{\mathrm{c}}}{\mu_{\mathrm{o}}}=\frac{\text { Speed of light in oil }}{\text { Speed of light in crystal }}$
Where, $\mu_c=$ absolute refractive index of crystal w.r.t vacuum.
$\mu_{\mathrm{o}}=\text { absolute refractive index of oil w.r.t. }$
vacuum.
${ }_{\mathrm{o}} \mu_{\mathrm{c}}=\frac{1.68}{1.2}=\frac{7}{5}=\frac{\text { Speed in oil }}{\text { Speed in crystal }}$
Fractional change in velocity,
When light travel from oil to crystal
$=\frac{\text { Speed in crystal }}{\text { Speed in oil }}=\frac{5}{7}$