You are Here: Home >< Physics

# Momentum of photon (mass=0) watch

1. I've been asked to work out the momentum of a photon, since they have a mass of zero, how would I do this, because I can't work out the de Broglie's wavelength.
I know p=mv, but m=0, hence mv=0, does this mean p=o too.
2. Use the following equations

c = f*lambda
E = hf
p = h/lambda
3. thanks very much
4. Does the mass of a photon really equal 0, though?
5. (Original post by Mathematician!)
Does the mass of a photon really equal 0, though?
Yes. Well zero rest mass.

So it still has momentum even without rest mass.
6. (Original post by TableChair)
Yes. Well zero rest mass.

So it still has momentum even without rest mass.
Don't even need to make it that complicated...its massless...so just E=cp
7. (Original post by -G-a-v-)
Don't even need to make it that complicated...its massless...so just E=cp
Energy = speed of light X momentum ?

Srry, how did u get this equation?

Thanks.
8. (Original post by -G-a-v-)
Don't even need to make it that complicated...its massless...so just E=cp
Yes I know, but I'm showing where it comes from.
9. (Original post by wizz_kid)
Energy = speed of light X momentum ?

Srry, how did u get this equation?

Thanks.
Two ways, either use the equations in my first post, or set in the the one I gave in my second.
10. (Original post by TableChair)
Yes. Well zero rest mass.

So it still has momentum even without rest mass.
In fact this is where the famous comes from; we assume the first term is negligible compared with the second (because of the relative powers of c).
11. (Original post by TableChair)
Yes. Well zero rest mass.

So it still has momentum even without rest mass.
Oh OK. That is interesting... What does "zero rest mass" actually mean? (What do you mean by rest? Completely stationary?)
12. rest mass is the energy associated with a body/particle that is not due to motion (i.e. the bit left over after you deduct kinetic energy).

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: November 12, 2008
The home of Results and Clearing

### 3,579

people online now

### 1,567,000

students helped last year
Today on TSR

Hang on, have Edexcel's come out already?

### University open days

1. SAE Institute
Animation, Audio, Film, Games, Music, Business, Web Further education
Thu, 16 Aug '18
2. Bournemouth University
Fri, 17 Aug '18
3. University of Bolton