What happens to the wavelength of light when it goes from rarer to denser medium?

What is the effect when light enters from a rarer medium to a denser medium? Explain with diagram

Can total inrternal reflection take place,when light travesl from rarer to denser medium?

What is the wavelength of light emitted when the electron in hydrogen atom undergoes transition from an energy level with n = 6 to an energy level with n = 3 ?

What is the wavelength of light emitted when the electron in a hydrogen atom undergoes trnsition from an energy level with n = 4 to an energy level with n = 2 ?