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Old 02-03-2005, 04:18 PM   #1 (permalink)
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Wave Physics

I know how a sound wave works: my vocal cords cause rapid pulses of pressure (or density) in the surrounding air molecules, those are amplified (increased in amplitude) inside the cavity of my throat and mouth, and those "waves" of pressure emanate from me until they are perceived by your ear and turned into sound.

In your High School physics book, sound waves were depicted as a sine wave, which is a simplification of what's actually going on. That sine wave is actually a graph of the pressures involved in the propagation of a sound wave.

Here's my question: Light waves were depicted the same way in my High School physics book, as a sine wave. But I'm not clear what the actual physical mechanism of light wave propagation is. I know that "light behaves as both a wave and a particle", but that doesn't give me much of an understanding.

Somebody who's up with physics answer this: HOW does light behave as both a particle and a wave? I understand there's such a thing as a photon, but I don't know quite what it is or how it works. And that's the particle side of the equation; tell me also about the wave part.

And then... tell me about light wave polarity. How do light waves work (again, in the real world, not in physics textbook simplifications) such that my polarized sunglasses filter out light that has been reflected?

Thanks!
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Old 02-03-2005, 06:06 PM   #2 (permalink)
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An example of how light behaves like a wave is how it interferes constructively and destructively with other light waves. Think of the popular slit experiment. A way light acts like a particle is how it travels in little packets of energy called quanta, like little balls of energy.

Try <a href=http://www.colorado.edu/physics/2000/polarization/polarizationI.html>this</a> for some help in understanding polarization.
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Old 02-03-2005, 06:31 PM   #3 (permalink)
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Light is electromagnetic waves. Maxwells equations tell us a changing electric field will cause a magnetic field, and a changing magnetic field will cause an electric field. In light waves, the electric field is changing in such a way that it causes a changing magnetic field... which makes a changing electric field, which makes a changing magnetic field, which makes... you get the point. These changing electric and magnetic fields form electromagnetic waves: light.
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Old 02-03-2005, 10:29 PM   #4 (permalink)
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Yes, and if I remember correctly from whatever presentation there was in my physics class last year, think of a masses photon, and it spins on an axis that it is moving. A line sticking out of the photon at an angle is the effect of the changing electric/magnetic changes. And if you remember good old kinematics like rotational motion! you remember that a spinning object when seen parallel to the axis of roation, is a SINE WAVE.

As for polarization, because the wave is actually spinning when it enters your polarized lenses: the lenses are design with a film that lets in only the "horizontal" part of the light. This is useful for making three-d goggles, one lense lets in one bit of light, the other lets in the other, and Wha-La! You have three dimensional illusions because your eyes are crossing towards the wrong point.
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Old 02-04-2005, 04:54 AM   #5 (permalink)
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Forget the wave/particle thing. It will only confuse you. In the vast majority of cases, light behaves like a wave. Photons are not particles in the sense that most non-physicists would interpret that word, and it is complicated to explain the differences.

The wavey nature of light shows up in interference and diffraction experiments. It also limits the capabilities of conventional microscopes and telescopes (in different ways). This even applies to CD/DVD/... technologies, where shorter wavelength lasers are needed to read the ever-shrinking markings. On the opposite scale, you have radio waves. These have very long wavelengths, which keeps them from being easily blocked by buildings or terrain.

Anyway, the thing that is "waving" to produce light is the electromagnetic field. This is an abstract thing that basically represents a potential to interact with charged particles (in a well-defined way). It may be represented by two vectors at each point in space - the electric and magnetic fields. These vectors are what are usually drawn in intro books.

If you have a very simple situation where the electric field is oscillating in a single plane, then the wave is said to be (completely) polarized. Usual light isn't (strongly) polarized, but is instead formed by a sum of waves oscillating in all different directions. Polarizing sunglasses filter out all of the waves oscillating outside of a particular plane. The reason that this is useful is that light becomes partially polarized when it is reflected. So the glasses are designed to filter out light bouncing off the ground. This is only effective at filtering out mirror-like surfaces, which is exactly what you'd want in a lens.

Also, don't think of waves too literally as sinusoids. Almost no real-life waves are ever pure sines. The reason that things are introduced that way is that it is simple to find the properties of waves with that shape. It also happens that arbitrary waves may be built up by summing together a number of different sine waves with different frequencies and amplitudes (called the Fourier transform). This is only possible because the equations governing the electromagnetic field are linear (at least outside of certain materials).
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Old 02-04-2005, 09:15 AM   #6 (permalink)
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Location: Troy, NY
Quote:
Originally Posted by ratbastid
Somebody who's up with physics answer this: HOW does light behave as both a particle and a wave? I understand there's such a thing as a photon, but I don't know quite what it is or how it works. And that's the particle side of the equation; tell me also about the wave part.
Man... This is going back a few years to my undergrad quantum class, but let me see if I can simplify this correctly...

Light in most circumtances as regular people will understand them (and have need to understand them) will act as a wave. The only thing about it being a particle is that a photon has a momentum, so when it strikes something that has mass (namely a subatomic particle and usually an e-), it can cause the subatomic particle move. Normally only things with mass can cause a force on something else with mass, but photons are an exception. Normally momentum is just the velocity vector times the mass of the object, but in photons it's measured by taking planck's constant and dividing by the wavelegnth of the photon. The units work out to be the same if you care to do the quasi-math.
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Old 02-04-2005, 02:35 PM   #7 (permalink)
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Location: PA
Actually, light has momentum even in the classical wave theory. The difference is that photons require that that momentum can only exist in discrete chunks.

Also, any classical field causes forces on massive objects. It wouldn't be detectable otherwise!
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Old 02-06-2005, 09:17 AM   #8 (permalink)
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Location: Ithaca, New York
I posted in another thread a couple months back about light and what light waves are, but now I can't find it

I'm sure you have seen the demonstration of magnetic force lines. I take a magnet, and place it bellow a peice of paper. Then, I sprinkle some iron fillings on the piece of paper. What do I see? I see that the iron fillings all line up, and make lines connecting one pole of the magnet with the other pole.

These lines are called lines of force. They behave a little bit like pieces of string. If I take a piece of string and I wiggle one end, that wiggle will travel down the piece of string to the other end. Similarly, if I wiggle the magnet, the force lines will also wiggle.

Now, instead of a magnet, we will use an electron. The force lines (aka, the Field) will look different than the magnet. Instead of connecting two ends, the force lines radiate outward from the electron like a child might draw rays radiating from the sun. These lines indicate the direction and magnitude that another charged particle would feel due to the presence of the electron.

If I grab that electron and wiggle it, the wiggle will travel outward along the force lines. The wiggle of the force lines is light. I can do the same thing with any charged matter, as long as I wiggle it or cause it to move non-inertially. Light is caused by the acceleration of charges.
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Old 02-07-2005, 01:28 AM   #9 (permalink)
 
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Location: Waterloo, Ontario
Quote:
Originally Posted by ratbastid
Here's my question: Light waves were depicted the same way in my High School physics book, as a sine wave. But I'm not clear what the actual physical mechanism of light wave propagation is. I know that "light behaves as both a wave and a particle", but that doesn't give me much of an understanding.

Somebody who's up with physics answer this: HOW does light behave as both a particle and a wave? I understand there's such a thing as a photon, but I don't know quite what it is or how it works. And that's the particle side of the equation; tell me also about the wave part.

And then... tell me about light wave polarity. How do light waves work (again, in the real world, not in physics textbook simplifications) such that my polarized sunglasses filter out light that has been reflected?
Hey, I used to wonder exactly the same things you are! At least you understand what a pressure wave is (like sound). I don't think it's any different for electromagnetic waves.

Suppose you have an object with a certain amount of charge. Assuming you understand what force fields are, that object will have an electric field around it, extending to infiinity (barring obstructions, of course). Note that the field is oriented. This reflects how the force caused by the charge must do so in a specific direction. This will be important, later. For now, we will examine what happens when we move the charged object (in some direction). The electric field will change and this change will propogate through space (can you guess at what speed?). Can you see how this propogation is a wave very similar to your pressure wave?

As far as I can tell, the only difference between this wave and a pressure wave is that this wave is oriented because the thing that's propogating (electric field strength) is oriented. It's this orientation that allows this wave to be polarized. If we examine a path of the wave (the electric field propogated in all directions) perpendicular to the movement of the object, you will see how the orientation of the field can be in one direction before the wave front and then the other direction after the wave front. Thus, the wave has an orientation in 3 dimensions. It turns out that the diagram in your book wasn't so far off, after all...

I hope this no nonsense explanation helped ground these abstract ideas into reality!
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