# Homework Help: Proving that a free electron can't fully absorb a photon

1. Oct 30, 2013

### squareroot

1. The problem statement, all variables and given/known data
Prove that a free, stationary, electron can't fully absorb a photon
2. Relevant equations

3. The attempt at a solution
I have no idea where to start.I must mention that i m on the 12 grade and not using calculus-based physics.And i've only studied and learned equations in quantum mechanics that involve planck's constant(h) and not the reduced planck constant (h bar).

Thank you!

2. Oct 30, 2013

### ehild

There are two conservation laws to be fulfilled when the electron absorbs the photon. What are they? Can be both of them true?

ehild

3. Oct 30, 2013

### squareroot

You're talking about the conservation of energy and the conservation of momentum.I presented this solution to my teacher and he rejected it.

Energy of the photon :Ef=pfc
Energy of the electron Ee=mec2

Now, the conservation of energy tells us that the final energy(after the absorbtion) is the same with the energy of the photon and the electron, so:

The total energy E=Ee + Ef

Am I right so far?Because this is the point at which i lose confidence

What's the total energy?
And where do I use the conservation of momentum?

Ty

4. Oct 30, 2013

### UltrafastPED

5. Oct 30, 2013

### ehild

The electron is in rest initially.
I think you can assume that the speed of the electron would be much below the speed of light, so you can say that the energy of the photon becomes the kinetic energy of the electron, and the momentum of the photon should be equal to the momentum of the electron after their interaction.
You can use he classical formula for the kinetic energy and the momentum of the electron. What are they?
What are the energy and momentum of the photon?

ehild

6. Oct 31, 2013

### squareroot

I solved it!

Thank you very much!

7. Oct 31, 2013

### ehild

You are welcome

ehild