# Recoil shift from a free H atom.

1. Jul 20, 2011

### atomicpedals

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

part b) Calculate the recoil shift for the emission of the most energetic Lyman line from a free atom of hydrogen.

2. The attempt at a solution

My question isn't a physics one per se, I'm stuck on terminology. What is meant by "the most energetic Lyman line"? I'm use to thinking of Lyman series in terms of frequency, and it doesn't seem to make sense to me to try and calculate the energy from frequency in this case. Any thoughts?

2. Jul 20, 2011

### Dick

Use Planck's constant. If v is the frequency of the emitted photon, then the energy E=hv, where h is Planck's constant.

3. Jul 21, 2011

### atomicpedals

Right, which is also E = hc/lambda. I guess my sticking point is in comprehension of what I'm being asked rather that the mathematics or arithmetic.

4. Jul 21, 2011

### Aimless

Do photons carry momentum?

What does conservation of momentum say about what happens to an atom at rest which emits a photon?

5. Jul 21, 2011

### Redbelly98

Staff Emeritus
Yes, so "most energetic" is synonymous with "shortest wavelength" or "highest frequency".

6. Jul 21, 2011

### atomicpedals

Ah, gotcha. The fog has been lifted, thanks for the help!