# Homework Help: Finding initial wavelength of photo given angle of deflection

1. May 28, 2014

### negation

1. The problem statement, all variables and given/known data

Find the initial wavelength of a photon that loses half its energy when it Compton-scatters from an electron and emerges at 90∘ to its initial direction of motion.

3. The attempt at a solution

Ei = Ef +KE
Ei = 0.5Ei + KE

Using compton's formula for scattering of photon:

Δλ = λf - λi = 2.42444x10-12 m

λi = λf - 2.42444x10-12 m

pretty much stuck. If I know the initia energy, find the initial wavelength would be a breeze but I don't have that information. Or if I know the numerical value of KE, I can by KE = 0.5Ei, find initial wavelength.

2. May 28, 2014

### BvU

What did you do with the 90 degrees information ?

 Oops, already used. Good. So what does halving the energy do to the wavelength of a photon ?

Last edited: May 28, 2014
3. May 28, 2014

### negation

not really sure if I will incur the wrath of the moderators but I already solved it.

The key was introducing E=mc2 and E = hf
one learnts that f could be found.
f is also equals to c/λ and with this λ could be found.

4. May 28, 2014

### BvU

Well done. No reasons for our good moderating spirits to get excited at all !