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Special relativity energy
A photon (no mass) moves into and collides with an electron. The photon has energy 1.25MeV. The electron was initially at rest. After colliding the photon rebounds in the opposite direction.
Show the relation between the photon momentum before the collision p' and after, p.
And show the mass of the electron to be given by:
p' = mcp/(2p + mc)
If the mass of the electron is 0.51MeV/c^2, find the energy of the photon after the collision. What are the energy, speed and momentum of the electron after the collision?
Positive rep for a solution to the first part, or both parts. I just have NO idea what to do about the fact that the photon has no mass... It makes no sense.
Show the relation between the photon momentum before the collision p' and after, p.
And show the mass of the electron to be given by:
p' = mcp/(2p + mc)
If the mass of the electron is 0.51MeV/c^2, find the energy of the photon after the collision. What are the energy, speed and momentum of the electron after the collision?
Positive rep for a solution to the first part, or both parts. I just have NO idea what to do about the fact that the photon has no mass... It makes no sense.
Well, it's nice to know someone else is grappling with this sort of stuff at this ungodly hour 
For p', that's given by E = cp'
p'e (initial momentum of electron) is 0 (but you knew that!)
pe is given by Ee ² = c²pe² + me²c4
since p' + p'e = p + pe
you can put these together to get an expression for p and (I hope!) answer the rest of the questions
For p', that's given by E = cp'
p'e (initial momentum of electron) is 0 (but you knew that!)
pe is given by Ee ² = c²pe² + me²c4
since p' + p'e = p + pe
you can put these together to get an expression for p and (I hope!) answer the rest of the questions
mik1w
Are you sure E=cp can be applied to a massive partice?
The condition E = pc implies null energy-momentum, so can only be true for massless particles.
As Ana syas all you need to do is manipulate the equation:
E2 = p2c^2 + m2c4
Inserting the values you already know and remembering which quantites are conserved.
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