What is the wavelength λ of light in glass, if its wavelength in air is λ0, its speed in air is c, and its speed in the glass is v? express your answer in terms of λ0, c, and v?

When an electromagnetic wave moves from a medium to another medium, its frequency remains constant. In this problem, we have light (which is an electromagnetic wave) moving from air to glass (or vice-versa), so we can write
(1)
where  is the frequency of the light in air, while f is the frequency in glass.

Using the relationship between frequency, wavelenght and speed of a wave, we have

where v is the speed of light in glass and  the wavelength in glass, and

where c is the speed of light in air and  the wavelength in air. Using these two relationships, we can rewrite eq.(1) as

and by re-arranging it, we find