If '$\mathit{c}$' is velocity of light in free space, the time taken by the light to travel a distance '$\mathit{x}$' in a medium of refractive index '$\mu$' is given by :

$(a)\;\mu \;x\;c \qquad (b)\;\frac{\mu\;x}{c} \qquad (c)\;\frac{\mu\;c}{x} \qquad (d)\; \frac{x}{\mu}$

$(b)\;\large\frac{\mu\;x}{c}$