When a monochromatic ray of light travels a medium of refractive index $n_1$ to a medium of refractive index $n_2\left(n_2>n_1\right)$ , its

When light travels from rarer to denser medium, the speed of light decreases by  factor of $\mu ,\left[\mu =\frac{n_2}{n_1}\right]$ 

wavelength also decreases. Frequency does not change as frequency depends only on source.