How do you calculate change in momentum?

Diffraction grating experiment to show photons have momentum?

• i was given a task to design an experiment that show photons have momentum using diffraction grating and also to calculate the momentum. The problem is, i'm kind of stuck in confusion here, and the deadline is almost due. photons carry energy and momentum, that is why they diffract. So when photons passes thru a diffraction grating, their momentum distributes? does their momentum change? I'm quite confused on how i can relate momentum of photon to a diffraction grating experiment. I srsly need help.. please? someone knock some physic sense into my head. =D

• Answer:

  This is a bit of a stretch, but... photons diffract since they are waves. We know they are supposed to be mass-less, but they could have momentum. This momentum would be in the direction of the wave. More momentum means less deflection. Using the diffraction grating equation (see link), you could compare the amount of diffraction of a photon to the amount of diffraction of an electron (since they, too, have a wavelength). You could also compare the width of the pattern that the electron makes to the photons'.