# Homework Help: Photon flux in Modern Physics

1. Feb 4, 2016

### Ashley1nOnly

1. The problem statement, all variables and given/known data
It's in attachment

2. Relevant equations

E=hf. Where h= planks constant and f= frequency
3. The attempt at a solution
I know that the flux of a photon is (# of photons)/(sec m^2)
I don't know the number of photos but I do know the frequency and power.

E=(6.6261*10^(-34)) J•s * (100 MHz)=6.621*10^-32
Also
Wavelength*frequnecy=c
I know that it's traveling 4 light years away.

Wavelength*(100MHz)=4c
Wavelength=4c/(100MHz

What equation am I supposed to use and how do I find the number of photon and how big in diameter would the recover/detector have to be. I don't want the answer just help understanding the problem and what I'm doing wrong. Or am I on the right track

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Last edited: Feb 4, 2016
2. Feb 4, 2016

### Staff: Mentor

The image has a wrong orientation.

You know the energy per photon, and the power. Can you get the photons per second, or per image frame?
To capture all photons in a distance of 4 light years, aliens would have to construct a sphere with a radius of 4 light years. They don't need all the photons, of course, but which fraction do they need?

3. Feb 4, 2016

### Ashley1nOnly

I know the
energy is 6.621*10^-32
power is 100Kw 24 frames per second.

what equation should i use to relate them

4. Feb 4, 2016

### Ashley1nOnly

I can do the power divided by the photon energy to get the number of photons arriving per second

100 KW/ 6.21*10^-32

5. Feb 4, 2016

### Staff: Mentor

Right.
As every second has 24 frames, how many photons are there per frame?

All those steps all don't need fancy physics.

6. Feb 4, 2016

### Ashley1nOnly

power divided by / ( the photon energy * 24) to get the number of photons per frame