Energy of a photon?

[Aki]

How would you calculate the energy of a photon? We cannot use E=mc^2 because it is massless. I found this equation E=hf , could I use this to find the energy of a photon then since the equation does not use mass? Photon behaves as a wave too, so it would have a frequency.

[TeleMad]

I found this equation E=hf , could I use this to find the energy of a photon then since the equation does not use mass? Photon behaves as a wave too, so it would have a frequency.

 

Yes, the f stands for frequency, so plug in the photon's frequency and multiply it by h, which is Planck's constant.

[Aki]

planck's constant is 6.626X10^ -34 J*s.

 

E=mc^2, the units would be kg*m^2*s^2

 

but with

E=hf, the units are J*s*cycle/s , or, kg*m^2*s^2*s*cycle/s

 

so there is an extra 'cycle' in the units, or can we just ignore that?

[TeleMad]

planck's constant is 6.626X10^ -34 J*s.

 

E=mc^2, the units would be kg*m^2*s^2

 

but with

E=hf, the units are J*s*cycle/s , or, kg*m^2*s^2*s*cycle/s

 

so there is an extra 'cycle' in the units, or can we just ignore that?

 

 

They are the same. The s's in the E=hf one cancel, so as you pointed out, you end up with:

 

kg*m^2*s^2 (for E=mc^2)

kg*m^2*s^2*cycle (for E=hf)

 

Hertz is often stated as being cycles/sec, but in calculations it is often times used as just sec^-1 (per second, without cycles).

[Bo]

in practice you evaluete a frequency for #cycles=1. (and the number of cycles is dimensionless)

as for the energy of the photon: see also the 1=2... thread; the same applies to a photon!

since with E=mc^2 we need to use the mass of the moving photon.

such a mass is given by: m = gamma*m0. Where gamma is a quantity that is 1/0 if the particle moves at the speed of light. So for a photon (m0=0), we have that

E=0/0 c^2. Or in other words: the energy is undetermined. We nee quantum effects to deermine the energy of a photon.

 

Bo

[BlameTheEx]

How would you calculate the energy of a photon? We cannot use E=mc^2 because it is massless. I found this equation E=hf , could I use this to find the energy of a photon then since the equation does not use mass? Photon behaves as a wave too, so it would have a frequency.

But photons are not massless! They have no rest mass, but that is of no account as they are never at rest.

 

The big problem is they won't sit still to be weighed. If you could put a photon in a bottle (say bouncing between 2 perfect mirrors) and you weighed the bottle the photons mass would be included.

[Qfwfq]

But photons are not massless!

Yes they are!!!

 

They have no rest mass, but that is of no account as they are never at rest.

 

The big problem is they won't sit still to be weighed.

If I were a photon, neither would I!!! :friday:

 

If you put a photon in a bottle of perfect mirrors and you weigh the bottle, the photon's rest energy would be included.

 

To a particle physicist, the expression "rest mass" means nothing. "Mass" has become an expression for "rest energy".

 

The photon between the mirrors adds mass to the overall system because it adds its energy to the system, hence increasing the rest energy. The photon is moving but the system's centre of mass isn't.

[Qfwfq]

the photon's rest energy would be included.

That should have been:

the photon's kinetic energy would be included.

As usual, posting in spare moments, nobody's perfect... :friday:

[BlameTheEx]

Qfwfq

 

A bit of confusion here. First you deny that a photon has mass, and then you agree that it does. I can only agree with you that a photon's mass is purely kinetic, but why should that not count?

 

It is more than likely that the mass of ALL particles is purely kinetic. They are probably standing waves. If "Mass" has become an expression for "rest energy" then it is high time we did something about it. How can we explain the effect of light on a solar sail without ascribing mass to photons?

[Qfwfq]

I'd say your confusion, BlameTheEx. :)

 

I did deny that a photon has mass but where did I agree it does? I can see that you replied to my rectification of my lapse in #7 and I can't see anywhere else that I contradicted myself. Ah, wait, yes, now I see... when you say:

...a photon's mass is purely kinetic, but why should that not count?

 

It is more than likely that the mass of ALL particles is purely kinetic. They are probably standing waves.

Shows that you did not follow my argument about how energy may or may not be called mass. The potential and kinetic energies of a composite body's parts contribute to it's rest energy i. e. its mass, but only that of the composite body. Simple.

 

If "Mass" has become an expression for "rest energy" then it is high time we did something about it.

What should we do about it? Spank most particle physicists on the butt, including the authors of just about all textbooks on the subject? If you don't choose to use Lorentz covariant terminology, that's your tough bananas.

 

How can we explain the effect of light on a solar sail without ascribing mass to photons?

We don't ascribe them a mass, we ascribe them energy and momentum: the usual 4-vector (E, px, py, pz). Its self contraction is e^2 - p^2 and is usually called the mass squared, a scalar quantity, hence independent of velocity: the same for all observers.

[BlameTheEx]

The potential and kinetic energies of a composite body's parts contribute to it's rest energy i. e. its mass, but only that of the composite body. Simple.

Hm. Maybe I need more sleep but that sentence seems far from simple to me. In fact if I didn't know better I would call it gibberish. I can't make head or tail of it. Anybody else out there who understands it?

What should we do about it? Spank most particle physicists on the butt, including the authors of just about all textbooks on the subject? If you don't choose to use Lorentz covariant terminology, that's your tough bananas.

Bananas? Well, I am glad that is cleared up then. Im wrong because of the bananas.

[geko]

The potential and kinetic energies of a composite body's parts contribute to it's rest energy i. e. its mass, but only that of the composite body. Simple.

 

To me this sounds like it's saying "the amount of rest energy in the parts of a composite body equal more than the potential and kinetic energies combined".

 

...but this is way out of my field :)

[Qfwfq]

Sorry if my sentence wasn't clear enough:

 

"The potential and kinetic energies of a composite body's parts contribute to it's rest energy i. e. its mass, but only that of the composite body."

 

OK, expand the sentnce by replacing the pronoun 'it' with 'the composite body' and 'that' with 'the mass' and here's what you get:

 

"The potential and kinetic energies of a composite body's parts contribute to the composite body's rest energy i. e. its mass, but only the mass of the composite body."

 

If you like, you can even add "and not to the mass of the parts" at the end. A bit less simple?

 

Im wrong because of the bananas.

Just as much as I was wrong because it was high time we did something about it. :)

[Qfwfq]

the amount of rest energy in the parts of a composite body equal more than the potential and kinetic energies combined

Not quite Geko, it's more like:

 

The amount of rest energy in a composite body equals more than the rest energies of the parts combined. This is so because we must add the potential and kinetic energies.

 

Unless of course the potential energy is negative compared with the asymptotic value, such as in the case of hydrogen fusing into helium. It can be 'more than' or 'less than'.