# Homework Help: Speed of a photon with wavelength = 500nm?

1. Aug 14, 2006

### delongk

What is the speed and momentum of a photon with wavelength 500nm?

I found momentum by wavelength = h/p but how do I find velocity?? Please help. Thanks. I don't know how to do it without a mass.

Kristin

2. Aug 14, 2006

### chroot

Staff Emeritus
All photons travel at c.

- Warren

3. Aug 14, 2006

### delongk

Well that makes it easy. Thank you tons for clearing that up... something so simple, you think they'd tell you that in the book. Thanks again!

4. Aug 14, 2006

### d_leet

Are you sure the question was about the speed of the photon and not the frequency? Since it seems rather odd to ask about speed, since as chroot said above all photons travel at c.

5. Aug 16, 2006

### Staff: Mentor

I laughed when I read this at first, but then I stopped laughing when I remembered that c varies. Quiz question -- why?

And then there's the extreme case that came up in a thread recently with the apparatus that stops photons completely. That was weird.

6. Aug 16, 2006

### inha

this is not correct. c does not vary. the apparent speed of photon propagation can vary in materials.

7. Aug 16, 2006

### Staff: Mentor

Good point. I misspoke in an effort to be brief. c is indeed a constant, but the speed of photons depends on the medium. Thanks for catching my error.

8. Aug 16, 2006

### DaveC426913

Actually, I think even that's oversimplifying.

The speed of photons is always c. However, they do not always "make headway" equivalent to c when in materials. (the photons lose headway when absorbed and re-emitted).

9. Aug 16, 2006

### Hootenanny

Staff Emeritus
Indeed, all photons travel at c; however, as has been said previsouly their rate of propogation can vary (depending on the medium). There is a related thread https://www.physicsforums.com/showthread.php?t=128221&highlight=photons+speed".

Last edited by a moderator: Apr 22, 2017