One of the problems in Rae's Quantum mechanics book, at the end of the first section, relates to x-ray's scattering off either an electron or a carbon atom. Using the equation given to you in the book I found that the higher the mass of the particle the x-ray is reflected off of the smaller the difference in wavelength.
Can anyone explain qualitatively why this happens?
It's exactly like in other collisions:
The more massive the thing a particle collides with is, the smaller is the change in absolute momentum of the incoming particle due to simultaneous energy and momentum conservation.
In ordinary collisions, this means the speed of the incoming particle varies with the mass of the target, and stays the same for an infinitely massive particle - light stuff just rebounds from heavy stuff.
But the photon has a fixed speed - the change in momentum here must correspond to a change in wavelength, and so the heavier the target is, the less the wavelength changes.
It should be connected to momentum conservation. A small difference in wavelength signals a small momentum change. A massive object responds less to a change in momentum with respect to an object with a smaller mass, hence the effect that you've observed. Of course, when the particles are hit with EM radiation, they must change their momentum (even if sometimes this change may be neglected).
