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

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The assumption that the wavelength of X-rays in a crystal is the same as outside is based on the approximation that the index of refraction for X-rays in most materials is close to 1. This means that the speed of electromagnetic waves, and consequently their wavelength, does not significantly change when entering the crystal. While it is true that the speed of electromagnetic waves can vary with different materials, this effect is minimal for X-ray wavelengths in typical crystals. Therefore, the approximation holds well for practical purposes in X-ray diffraction studies. Understanding this concept is crucial for accurate analysis in crystallography.
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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?
 
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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.
 
Thread 'Unexpected irregular reflection signal from a high-finesse cavity'

Thread 'Unexpected irregular reflection signal from a high-finesse cavity'

I am observing an irregular, aperiodic noise pattern in the reflection signal of a high-finesse optical cavity (finesse ≈ 20,000). The cavity is normally operated using a standard Pound–Drever–Hall (PDH) locking configuration, where an EOM provides phase modulation. The signals shown in the attached figures were recorded with the modulation turned off. Under these conditions, when scanning the laser frequency across a cavity resonance, I expected to observe a simple reflection dip. Instead...
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