A ray of light of wavelength λ0 and frequency v0 enters a glass slab of refractive index μ from air. Then

Whenever a ray goes from one medium to another, its frequency remains same. Its wavelength changes in accordance with μλ= constant. i.eμ0λ0=μλ    (∴μ0=μair=1)λ0=μλλ=λ0μ(<λ)So, λ decreases when it goes rarer to denser medium and vice versa.Δλ=λ−λ0Δλ=λ0(1μ−1)Δλ=λ0(1μ−1) and Δv=ν-ν=0