# Confusion regarding light [duplicate]

I have two question about light:

1. When a light wave travels from free space to a medium then there is a change in the amplitude. Why?

2. when a wave changes its medium then its frequency does not change but wavelength changes. Why?

2. The wavelength changes because the change in the medium usually means a change in refractive index $n$. The phase velocity $v$ of the wave is $v=c/n$ where c is the speed of light in vacuum. Therefore, when the wavelength in medium 1 is $\lambda_1=c/n_1$, the wavelength in medium 2 will be $\lambda_2=c/n_2$. Thus $$\lambda_1/\lambda_2=n_2/n_1$$