The refractive index of the material of crystal is 1.68 and that of castor oil is 1.2. When a ray of light passes from oil to glass, its velocity will change by factor

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We know, refractive index of crystal w. r. t. oil,
$μ_{c o} = \frac{μ_{c}}{μ_{o}} = \frac{s p e e d o f l i g h t i n o i l}{s p e e d o f l i g h t i n c r y s t a l}$
Where, $μ_{c}$ = absolute refractive index of crystal w. r. t. vacuum.
and $μ_{0}$ = absolute refractive index of oil w. r. t. vacuum.
$μ_{c o} = \frac{1.68}{1.2} \frac{7}{5} = \frac{s p e e d i n o i l}{s p e e d i n c r y s t a l}$
Fractional change in velocity,
when light travel from oil to crystal
$= \frac{s p e e d i n c r y s t a l}{s p e e d i n o i l} = \frac{5}{7}$

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