A light of wavelength $6000Å$ in air enters a medium of refractive index $1.5$. Inside the medium, its frequency is $\nu$ and its wavelength is $\lambda$. 

According to Snell’s Law, we have 

${\mu }_1{\lambda }_1={\mu }_2{\lambda }_2$

$\Rightarrow \left(1\right)\left(6000\right)=(1.5){\lambda }_2$

$\Rightarrow {\lambda }_2=4000Å$

Since frequency does not change when light goes from one medium to another, so 

$v=\frac{c}{{\lambda }_1}$

$\Rightarrow v=\frac{3\times {10}^8}{6000\times {10}^{-10}}=5\times {10}^{14} \mathrm{Hz}$