# confused - photons having diff energies?

Discussion in 'Physics & Math' started by nanok, Jan 29, 2006.

1. ### nanokRegistered Senior Member

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30
Hello all, I'm in the process of reading a few books on quantum mechanics and have come to a few points that I don't fully understand. Hopefully some of you guys can clear this up.

Ok quickly stated, photo-electric effect shows how light can act as a particle because of the fact that there is a threshold at a certain higher energy light that can change the charge of the plate (ie UV light works and visible light doesn't). But I am confused on the core concept of how photons can have higher or lower energies. If the speed of light (photons) is constant and they are known to have no mass, then how can they have different energies?

3. ### Physics MonkeySnow Monkey and PhysicistRegistered Senior Member

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869
The energy of a photon is proportional to its frequency and has little to do with classical notions of mass and velocity. As a consequence, UV photons have a higher energy than visible photons. You should think of the photon as a fundamental excitation of the electromagnetic field, and in this respect it behaves rather like a harmonic oscillator. In the case of the quantum harmonic oscillator, the energy of the oscillator is proportional to the frequency of the oscillator.

5. ### catoless hate, more scienceRegistered Senior Member

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2,959
well said fellow monkey =]. E=hf (E=energy, h=planks constant, and f=frequency)

7. ### James RJust this guy, you know?Staff Member

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37,093
nonok:

The relevant equation for photon energy is E=hf, where f is the frequency and h is Planck's constant.

I am curious as to how you can study the photoelectric effect without being aware of that equation...

8. ### nanokRegistered Senior Member

Messages:
30
Thanks, I guess I was reading all about quantum mechanics and wave particle duality but not applying it at all to my classical idea of photons (which are well within the quantum realm).

And to answer your question James, the books I'm reading are popularized accounts of quantum mechanics, its almost standard practice for these books to focus on the concepts and shun most of the mathematics aside from E=mc2. Possibly when I'm finished with these books I'll dive into some of the equations that power the concepts, but for now I'm just getting to know the gist of it all.