A monochromatic beam of light travelling in vacuum enters a medium of refractive index $n$. What is the relation between the wavelengths of the incident and the refracted light waves?

Define wavefront. How is it related to the direction of the corresponding rays?

When monochromatic light is incident on a surface separating two media, both the reflected and the refracted light have same frequency as the incident light. Why?

When light travels from a rarer to a denser medium, it loses speed. Does this loss imply a reduction in energy carried by the light waves?

What is the geometrical shape of the wavefront for the portion of wavefront of light from a distant star intercepted by the earth?