Why is the wavelength in a crystal assumed to be the same as outside?

[Lojzek]

I read an introductory text about diffraction of X-rays in crystals and I am not sure why the wavelength in the crystal is assumed to be identical to the wavelength outside the crystal. I think that the speed of electromagnetic waves can depend on the type of matter, so wavelenghts can be different.
Can someone explain this?

 

[Pete99]

I guess they approximate the index of refraction to be 1. I think for most materials at x-ray wavelengths it is a good approximation.