# Homework Help: Fraction of lost energy in compton scattering

1. Jul 9, 2016

### Magnetic Boy

1. The problem statement, all variables and given/known data
After undergoing through 90° compton scattering, the fraction of energy lost by photon is
a) 10%
b) 20%
c) 50%
d) zero
e) none of these

2. Relevant equations
∆λ= h/moc (1-cosΦ)

3. The attempt at a solution
What i m doing is that, i get scattered photon energy and subtracting it from the total and dividing by the total. But it seems unsolvable as wave length of the photon before scattering not given

2. Jul 9, 2016

### malawi_glenn

what is the relation between the Energy of a photon and it's wavelength?

3. Jul 9, 2016

### malawi_glenn

If the energy of the photon before the scattering is denoted E and the energy of the photon is denoted E' what is the expression for "the fraction of energy lost"

4. Jul 9, 2016

### Magnetic Boy

Is there any such relation?? I don't know...

5. Jul 9, 2016

6. Jul 9, 2016

### malawi_glenn

but to answer the question. Look at the numbers given, they are really "nice". No way those can be reproduced by the scattering formula.

7. Jul 10, 2016

### Magnetic Boy

Got the eqn. But it need to have scattered energy frequency. Which is not given in the problem. Does this mean the option "none of above is correct"???

8. Jul 11, 2016

### James R

For a photon (light), frequency and wavelength are related.

9. Jul 11, 2016

### Magnetic Boy

Yes. But neither of them is given

10. Jul 11, 2016

### Delta²

Frequency and Energy of photon also are related E=hf where h plank's constant.

11. Jul 11, 2016

### Magnetic Boy

QUOTE="Delta², post: 5517626, member: 189563"]Frequency and Energy of photon also are related E=hf where h plank's constant.[/QUOTE]
I know it very well. But look at the question. Only angle of scattering is given. Does not it mean that we cannot find fraction of lost energy? I just want to confirm.. (or is there some way to find the fractional lost energy)

12. Jul 11, 2016

### Delta²

Seems to me you are right, we have to know the wavelength (or frequency) of the photon before scattering.

13. Jul 11, 2016

### dpopchev

Should not have the need for initial energy: https://www.hep.wisc.edu/~prepost/407/compton/compton.pdf

EDIT: made a calculation mistake, have no idea how to approach this questions

Last edited: Jul 11, 2016
14. Jul 11, 2016

### James R

Just to check, what equation did you get?

15. Jul 11, 2016

### Magnetic Boy

I got ΔE/E= (E'/mc2)(1-cosΦ)

16. Jul 12, 2016

### dpopchev

17. Jul 12, 2016

### James R

That looks right.

I agree that you'd need to know the wavelength or frequency of the incoming photon in order to get a numerical answer.

18. Jul 12, 2016

### Magnetic Boy

Thanks. So the answer is "none of these". Now i am sure. Some one answered it 50%. And i were really confused about that.

19. Jul 13, 2016

### James R

Well, I supposed it could be 50%, or 10% or 20%, if the incoming photon energy was whatever is necessary to get those values. We know it can't be zero, because incoming photon must lose energy if it is scattered at any angle other than 0 degrees. Your formula has $\Delta E/E = E'/mc^2$, where $\phi=90^\circ$, and $E'$ can't be zero.

Really, there should be an "(a), (b) or (c)" option.