The refractive index of a certain glass is $1.5$ for light whose wavelength in vacuum is $6000\AA$. The wavelength of this light when it passes through glass is

The correct option is A

4000 Å

we know velocity (v) = frequency $\times$ wavelength$\left(\lambda \right)$. In any medium frequency does not change since it depends on source.

$\frac{{\mu }_g}{{\mu }_v}$ = $\frac{{\lambda }_v}{{\lambda }_g}$

${\lambda }_g$ =$\frac{{\lambda }_v}{{\mu }_g}$ = 4000 Å