E = hv and also 0.5mv**2
For EM radiation
E = hv and also 0.5mc**2
Does light have mass? If not what is it?
Established Member
E = hv and also 0.5mv**2
For EM radiation
E = hv and also 0.5mc**2
Does light have mass? If not what is it?
Moderator
light has no mass, but it has momentum
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Bi-weekly space physics research "blog" at tusenfem.blogspot.co.at
Established Member
P = mvOriginally Posted by tusenfem
P = 0 x v
= 0
Banned
No...
momentum p = E/c = hf/c = h/lambda, where h is Planck's constant, f is the frequency, and c is, well, c! (speed of light). From: E=hf (Max Planck's black-body radiation equation).
Moderator
My bold, which comes from the full equation for energy:
E2 = p2c2 + m02c4, which can be derived somehow. Look it up in a basic physics book to get to know the somehow.
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Banned
I was disagreeing with you, tusenfem. I was disagreeing with the Newtonian equation for momentum being applied to relativistic velocities.
Oops! I meant, I "wasn't" disagreeing..." Sorry!
Last edited by mugaliens; 2009-Oct-07 at 09:34 AM.
Moderator
Well, yeah, relativistic is not Newtonian.
Newtonian momentum: m v
Relativistic momentum: m v = γ m0 v
Relativistic total energy: E2 = p2c2 + m02c4
As a photon has no m0 this reduces to: E2 = p2c2
As the energy of a photon is: E = h ν (which is btw not "Max Planck's black-body radiation equation")
One can get the momentum of a photon by: p = E/c = h ν / c
Methinks you were disagreeing with rodin.
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Bi-weekly space physics research "blog" at tusenfem.blogspot.co.at
Banned
Yes.
Order of Kilopi
And note that there's a good reason why you can't just use the relativistic formula p = γ m0 v for a photon. It's true that m0 is 0, which would lead you to think that the momentum must be zero, but gamma is infinite, which means the value of this expression is indeterminate.
Conserve energy. Commute with the Hamiltonian.
Banned
Exactly. Fortunately, Planck's equation is determinate.
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