refraction gcse

in the revision guide, it says that the amount that a wave refracts depends on its change of speed as it crosses a boundary due to density change

but it also says that how much a wave refracts depends on the wavelength for example short wavelengths bend more like the diffferent wavelengths in white light which disperse

so which one is it? change in speed or the wavelength
also if v=f x wavelength
then when freq is constant a lower speed means a shorter wavelength so surely waves travelling at a lower speed would bend less or does it bend more because of a shorter wavelength?

thanks

Reply 1

sigmagrindset49

Technically it is both.

The primary reason is due to the change in refractive index, which causes a change in speed. However the refractive index of a substance is different for different wavelengths of light. This change in refractive index is small but it exists. The refractive index of a substance is higher for shorter wavelengths of light. Therefore, shorter wavelengths of light do refract more due to slowing down more.

For example, when we say that the refractive index of a substance is 1.5, that is really only an approximation of the refractive index. The actual refractive index might be something like 1.503 for red light and 1.507 for blue light.

So in conclusion, the actual reason for refraction is the change in speed of light, and it just happens to be the case that shorter wavelengths of light slow down more, because the refractive index of substances are actually higher for shorter wavelengths of light.