# Can somebody help me this interference question

Light of wavelength 750.0 nm is sent along a route of length 2020 nm in vacuum. The route is then filled with a medium having an index of refraction of 1.400. In degrees, by how much does the medium phase shift the light? Give the full shift and then the equivalent shift which is between 0° and 360°

I have no idea how to start this question.

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Light of wavelength 750.0 nm is sent along a route of length 2020 nm in vacuum.
How many wavelengths would span this distance? (Include the fractional part!)

The route is then filled with a medium having an index of refraction of 1.400. In degrees, by how much does the medium phase shift the light?
What does a medium with an index of refraction n do to the wavelength of light traveling through it? How many wavelengths within the medium would now span the 2020 nm distance?

Give the full shift and then the equivalent shift which is between 0° and 360°
The full shift would be the difference in these answers multiplied by 360º. How do you then find the "equivalent shift"?

BTW, this problem as it stands does not involve the concept of wave interference. (They're probably setting you up, though, for some that do...)