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Title: GR8677.045
Post by: conantee on January 02, 2010, 02:00:53 PM
45. Photons of wavelength \lambda scatter elastically on free protons initially at rest. The wavelength of the photons scatterd at 90^o is increased by
(A) \lambda /137
(B) \lambda /1836
(C) h/m_e c, where  h is Planck's constant, m_e the rest mass of an electron, and c the speed of light.
(D) h/m_p c, where  h is Planck's constant, m_p the rest mass of a proton, and c the speed of light.
(E) zero.


Title: Re: GR8677.045
Post by: conantee on January 02, 2010, 02:05:13 PM
I got the solution from the grad student who also taught this course. It's quite funny. The answer is (D) since it is the only answer which involves with proton. That's it.

For more words, we know that this is just phenomenon called Compton scattering, but with proton instead of electron. Hence, even though you don't remember the equation for wavelength of photon after collision exactly, you should know that it depends on the mass of target particles. So, the only choice that makes sense is (D).

The correct answer is (D), and 41 of 100 people answer the right one.

Physics behind it: check 'Compton scattering'. You can plug values in and check if the answer (D) is actually correct or not.