# Homework Help: Electromagnetic radiation and photons

1. Jun 1, 2008

### freefallin38

[SOLVED] Quantum Theory questions...

I have been having a bit of trouble on these homework questions

1)How many photons/s are contained in a beam of electromagnetic radiation of total power of 165 W when the each of the following is the source.
(a) AM radio station of 990 kHz
_____photons/s
(b) 5 nm x-rays
_____photons/s
(c) 4 MeV gamma rays?
_____photons/s

2)An experimenter finds that no photoelectrons are emitted from a particular metal unless the wavelength of light is less than 270 nm. Her experiment will require photoelectrons of maximum kinetic energy 2.2 eV. What frequency light should be used to illuminate the metal?
_____Hz

3)A photon having 64 keV scatters from a free electron at rest. What is the maximum energy that the electron can obtain?
_____keV

4) What is the minimum energy that photons must have to produce 46 keV electrons in a Compton scattering?
_____keV

I know 3 and 4 are similar.. I wasn't sure about what kind of formulas to use for any of them . Any hints or suggestions would be amazing

2. Jun 1, 2008

### Dick

How about an equation that might express the energy of a photon in terms of it's wavelength or frequency? You really don't have ANY such formula?

3. Jun 2, 2008

### freefallin38

is E=h*f the energy for one photon?

4. Jun 2, 2008

### Dick

Yes, now get started.

5. Jun 2, 2008

### freefallin38

ok, so now I have the answers for 1 and 2. For 3 and 4 I'm still stuck about how to solve them..

6. Jun 2, 2008

### G01

Think of this collision as elastic. What does this mean?

HINT: Think "conservation laws."

7. Jun 2, 2008

### freefallin38

ok, so the conservation law is hf+mc^2=hf'+E(electron)
in problem 3, I have 64kEv+mc^2=hf'+E, but how do i find the f'?
I have a feeling that if i can get 3, 4 will be easy to solve

8. Jun 2, 2008

### Dick

Momentum is also conserved.

9. Jun 2, 2008

### freefallin38

I can't find f' for problem 3. I thought that because the problem said it wanted the maximum value, that would make $$\vartheta$$ =180. Using the formula 1/f'-1/f=h/mc*(1-sin$$\vartheta$$) to find f' and then when I used the conservation law, I just got the same original value of 64keV. If $$\vartheta$$ isn't 180, then I can't figure out how to find f' when $$\vartheta$$ and $$\phi$$ are also unknown?