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Old Mar 1st 2013, 03:15 PM   #1
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Frequency and Wavelength

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Originally Posted by muon321 View Post
So, let's say we have a proton, and it is at rest, and we use the equation λ=h/p to find it has a wavelength of 0 because it has no momentum. Makes sense. Then we also use the equation E^2=m^2c^4+p^2c^2 to find its energy from its rest mass, then we plug that into E=hv to find that it still has frequency. But, shouldn't it not have frequency because it has no wavelength? If we use fλ=v_p, we get 0 for frequency. Someone help! I'm confused!

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Old Mar 1st 2013, 04:04 PM   #2
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Originally Posted by muon321 View Post
So, let's say we have a proton, and it is at rest, and we use the equation λ=h/p to find it has a wavelength of 0 because it has no momentum. Makes sense. Then we also use the equation E^2=m^2c^4+p^2c^2 to find its energy from its rest mass, then we plug that into E=hv to find that it still has frequency. But, shouldn't it not have frequency because it has no wavelength? If we use fλ=v_p, we get 0 for frequency. Someone help! I'm confused!
E = h(nu) only works for massless particles.

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Last edited by topsquark; Mar 2nd 2013 at 06:18 PM.
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Old Mar 2nd 2013, 06:09 PM   #3
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0muon321 has pointed out that my last post is incorrect. He is quite right. The equation I was going for is E = hc/(lambda).

So, let's say we have a proton, and it is at rest, and we use the equation λ=h/p to find it has a wavelength of 0 because it has no momentum.
Actually there is a problem on this line. (lambda) = h/p runs into a problem when v = 0. (lambda) is undefined for a particle at rest, due to p = 0.

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Last edited by topsquark; Mar 2nd 2013 at 06:25 PM.
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Old Mar 5th 2013, 10:46 PM   #4
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Originally Posted by topsquark View Post
E = h(nu) only works for massless particles.

-Dan
That is incorrect. E = hf holds for all particles. This is the de Broglie hypothesis.

Don't worry. I confirmed this with two different QM texts, one of which is my graduate text Quantum Mechanics by Cohen-Tannoudji et al
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