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Old 06-25-2010, 03:25 PM #1
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Default How can I understand Light as a Wave?

Ppl on Physics forums tell me that calling it a particle is wrong & that "wave" is just an analogy. Its not a mechanical wave like the text books I have show the object traveling up/down. So if its not traveling in a physical wave up & down, than what is meant by the wavelength? & frequency?

The wiki says a bit about the photon being an oscillating wave of electric and magnetic fields. I have a hard time understanding that.

I think these analogies are the best we can do, other than study the mathematics in qm? So its probably pointless for me to try & understand it.

Any advice? My knowledge is pretty limited, but I find light fascinating.


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Old 06-25-2010, 03:26 PM #2
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Default Re: How can I understand Light as a Wave?

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Old 06-25-2010, 04:11 PM #3
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Default Re: How can I understand Light as a Wave?

Unless you are in some pretty advanced physics you kinda have to take it on faith. All fields have associated particles but depending on what you are measuring determines whether a photon acts like a wave or particle. You are basically standing on a quantum cliff. Once you step off there is no going back. To get a better understanding study de Broglie, Heisenberg, Einstein (photo electric effect), Planck, Maxwell, Boltzmann, Schrödinger, Dirac, and others.

As you can see, it requires understanding quite a bit to build a mental picture of how it all works together. Furthermore, you will need a good understanding of calculus including Tensor mathematics, trigonometry with emphasis in identities as well as statistical analysis and probability. Physical analogies do not do the actual processes any justice but they can help to a degree. Just don't rely on the analogy too much as it can limit your ability to transcend or see beyond the analogy.

One of the nice things about quantum physics is that much of the study can be applied to any sub-field once the foundation is laid. If you study nuclear physics, for example, you gain a very good understanding of light because they all obey the laws of quantum physics.
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Old 06-25-2010, 08:56 PM #4
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Default Re: How can I understand Light as a Wave?

Its not pointless to try and understand it. You don't need a comprehensive understanding of physics, and light to understand that light travels in a wave.

To be VERY straight forward, I just think of light as a disturbance. In space, charges can form an electric field, and a magnetic field. Its important that you can at least visualize either one of those to get started.

Light, as a "disturbance" in an electromagnetic field (google that term if you have not already), can then gain the properties of a wave, such as a period, frequency, wavelength, and propagation speed. BUT in some situations it's very important to remember that these waves can also be thought of as very small bundles of waves. This introduces the idea that energy can be radiated or absorbed in small bundles of energy at a time. As a bundle, waves will act like a particle... with wave-like properties.

That's how you get a photon (particle like quanta), with a wavelength.

Do research on the men that Frothy listed off, they are the fathers of quantum mechanics. Learning what they discovered and how they discovered it will tell you a lot.
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Old 06-25-2010, 09:08 PM #5
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Default Re: How can I understand Light as a Wave?

Just to clear a little up, light can act as a particle or a wave... Just one of the puzzles of the universe.

so waves can diffract, like so:



I finding thinking of it like water helps. So that's one way it can be a wave, where as if it was a particle (a photon) it wouldn't be able to diffract, there for it must have wave properties.

But, this is where it gets confusing, in the explaining of the photoelectric effect, you must use the particle side of things (photons). Look up photoelectric. Basically electrons can be released as free electrons and transfer current through a gap using the energy transfered from photons. So the photons collide with the metal plate and transfer their energy to an atom of the metal, specifically one of it's electrons around the nucleus, this energy then allows the electron to escape the atom as it has so much energy and free it's self from the metal surface.



Different colours of light have more or less energy, the lower wavlengths of light (violet) carry more energy, so the photons also have more energy, where as red light (longer wavelength) has less energy per photon. So you have to have a minimum amount of energy to release the electron from different metal surfaces, this is called the threshold frequency.

E=hf where h is plancks constant, ( a set number) allows you to work out how much energy each frequency of light has, so you an work out how much energy a photon has at a certain frequency of light.

Hopefully that gives you a brief insight into the world of physics lol, hopefully soon enough I'll learn more (17 years old) when I head off to university next year.
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Old 06-26-2010, 12:22 AM #6
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Default Re: How can I understand Light as a Wave?

Thanks for the replies. It helps.
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Old 06-26-2010, 08:14 AM #7
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Default Re: How can I understand Light as a Wave?

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Thanks for the replies. It helps.
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Old 06-27-2010, 04:10 AM #8
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Default Re: How can I understand Light as a Wave?

Hey guys... question... First I just want to make sure this assumption is correct- does light alternate between an electric and magnetic field? Like does the electric field collapse and the magnetic field grow, and then vice versa?
thanks!
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Old 06-27-2010, 05:21 AM #9
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Default Re: How can I understand Light as a Wave?

Most typical illustration, notice the E & M vectors at each end, makes it a little easier to see, I think. I leave it as an exercise to the reader to determine which color is the E field and which color is the M field.



Also, I've read in at least 1 place that the phase relationship between E & M components in this illustration might not be exactly right. I've never cared enough to really look that much into this particular aspect of an electromagnetic wave, since it's just a demonstration animation to help you think about iut and not anything that important to me, but you can feel free.

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Old 06-27-2010, 05:29 AM #10
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Default Re: How can I understand Light as a Wave?

so the E and M fields are both strongest at the same time? Wouldn't that violate thermodynamics or something, all the energy goes away, or depending on how you look at it all the energy comes out of nothing? I was under the impression one grew as the other collapsed to avoid violating any laws.
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Old 06-27-2010, 05:38 AM #11
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Default Re: How can I understand Light as a Wave?

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so the E and M fields are both strongest at the same time? Wouldn't that violate thermodynamics or something, all the energy goes away, or depending on how you look at it all the energy comes out of nothing? I was under the impression one grew as the other collapsed to avoid violating any laws.
Some will argue that, but you need to solve Maxwell's equations to see it: E at a maximum when dB/dt is at a maximum doesn't make much sense once you look at the math.

For a plane wave in vacuum, I believe the 2 should be in phase, but again, I've never done the math to prove this to myself. My understanding is that, as far as EM waves in a medium, if there is an imaginary component in the index of refraction, then the E and M components are out of phase, and you get loss or gain.

Also, thermodynamics cannot be applied to this. While conservation of energy is an important consideration in thermo, it is much larger than thermodynamics, indeed universal, where thermodynamics is not. Conservation of energy does always apply, but the laws of thermodynamics do not apply.

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Old 06-27-2010, 05:46 AM #12
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Default Re: How can I understand Light as a Wave?

I suppose that makes sense. I still have to take some of these classes, maybe some physics class will fall over my free period next year and I can learn some more of this stuff. Second question: you couldn't do the same thing with light as you can do with with noise-canceling headphones, can you? Eg. put the waves 180º out of phase and have them neutralize each other(destructive wave interference)? It seems like that might violate a few laws...
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Old 06-27-2010, 06:03 AM #13
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Default Re: How can I understand Light as a Wave?

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I suppose that makes sense. I still have to take some of these classes, maybe some physics class will fall over my free period next year and I can learn some more of this stuff. Second question: you couldn't do the same thing with light as you can do with with noise-canceling headphones, can you? Eg. put the waves 180º out of phase and have them neutralize each other(destructive wave interference)? It seems like that might violate a few laws...
Destructive interference does happen with light, definitely, hence diffraction gratings and many other effects.

But a device like "light-canceling sunglasses" in the same vein as noise-canceling headphones would be very difficult, because of many physics problems, such as working around the speed of light. The light is always moving at c/n (essentially c in air), but you have to detect the light, analyze it, and then emit new light, all while the original wave hasn't passed by yet, and yet none of your signals can travel faster than light. Easier with sound, which moves slow allowing electronic signals to move faster, so you can "get ahead" of the sound wave. Make sense?
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Old 06-27-2010, 06:10 AM #14
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Default Re: How can I understand Light as a Wave?

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Destructive interference does happen with light, definiltely, hence diffraction gratings.

But a device like "light-canceling sunglasses" in the same vein as noise-canceling headphones would be very difficult, such as working around the speed of light. The light is always moving at c/n (essentially c in air), but you have to detect the light, analyze it, and then emit new light, all while the original wave hasn't passed by yet, and yet none of your signals can travel faster than light. Easier with sound, which moves slow allowing electronic signals to move faster, so you can "get ahead" of the sound wave. Make sense?
I don't see how that would work, why would adding light to another light source (out of phase) equal no light? 1 watt+1 watt=0 watts? I feel like that would violate certain thermodynamic laws. actually, if this were possible, couldn't you determine that the net energy of a photon is zero? Think about it, you could split a laser beam and then bounce half around in some kind of optic until the two beams are exactly 180º out of phase and then recombine them. Bam, energy gone. WTF happened there? Actually, come to think of it, isn't the net energy of every wave zero if you think about it like that? I must be missing something here... something huge LOL.
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Old 06-27-2010, 06:31 AM #15
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Default Re: How can I understand Light as a Wave?

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I don't see how that would work, why would adding light to another light source (out of phase) equal no light? 1 watt+1 watt=0 watts? I feel like that would violate certain thermodynamic laws. actually, if this were possible, couldn't you determine that the net energy of a photon is zero? Think about it, you could split a laser beam and then bounce half around in some kind of optic until the two beams are exactly 180º out of phase and then recombine them. Bam, energy gone. WTF happened there? Actually, come to think of it, isn't the net energy of every wave zero if you think about it like that? I must be missing something here... something huge LOL.
Here's a good explanation: Suppose 2 sinusoidal waves undergo totally destructive interference. This results in wave with zero amplitude (and hence zero intensity). What happens to the energy associated with wave (1) and wave (2)?

Basically, the energy is somewhere else. Like the simple example of a diffraction gratings, the constructive interference/peaks cancel out the energy of the destructive interference/troughs. The sources can also absorb energy.
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Old 06-27-2010, 06:41 AM #16
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Default Re: How can I understand Light as a Wave?

ahh! There we go. Now I get it (at least with sound and stuff). But what happens with laser light? If the two beams intersect in space, they cancel each other out and the energy is not absorbed by anything... Additionally, if you took two green laser pointers and put the beams together, would they not interfere (constructively or destructively) and change their frequency (as frequency is linked to amplitude)? Wouldn't you end up with different color light?
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