When a light enters glass from vacuum, then the wavelength of light decreases. This is because the speed of light in glass is less than that in the vacuum. From Snell's law, the refractive index in a medium is given by
$\mu=\frac{c}{v}$
The glass is of higher refractive Index than , Vacuum So speed of light decreases and as the speed of light is directly proportional to the wavelength $\left(c = v \lambda\right)$, so, the wavelength also be decreases