# Homework Help: Photon with negative energy, Compton Scattering

1. Mar 23, 2013

### zellwwf

Hello PF people,

1. The problem statement, all variables and given/known data
In a Compton scattering event, after the collision, the Photon has an energy of 0.12 MeV
and the Electron has an energy of 0.04 MeV. Find the following:

i) The Wavelength of the photon before the collision.
ii) The scattering angle for the photon after the collision.
iii) The scattering angle for the electron after the collision.

2. Relevant equations

1 . Conservation of Energy
2 . Conservation of Momentum
3 . Compton's Scattering $Δλ = \frac{h}{mc}(1-cos(θ))$

3. The attempt at a solution
I tried to equate the conservation of energy,
i get a negative energy for the photon before it collides:
E_p + E_e = E_p + E_e
Then substituting (in MeV):
X + 0.511 = 0.04 + 0.12 --> X = -0.351 MeV

I did not copy anything wrong, the problem sheet gives these numbers, what do i do? is this wrong or should i continue?

2. Mar 23, 2013

### voko

The given energy for the electron obviously does not include its rest energy.

3. Mar 23, 2013

### zellwwf

Hello again voko, ur my savior! :)
Yea, what you are saying makes total sense.. thank you a lot