When light travels from the air to the glass: 

• We know that, 
  $\mathrm{v} = \mathrm{f\lambda }$
  Where v is the speed of light, f is the frequency and $\lambda$ is the wavelength. 
• When a ray of light enters from air to glass, the speed of light decreases due to the higher refractive index of glass. 
• Since the frequency of a given colour of light always remains constant, the wavelength of the light must decrease.

Therefore, the wavelength of the light will decrease.