Light is a wave?

hockeyguy
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Topic 189787

in physics class we learned that a wave is just individual atoms moving up and down. it was explained to me as similar to doing the wave at some sports venue. The only motion is individual ppl standing up and sitting down. The observed motion is a left to right movement.

sound waves hold true to this, they are air molecules moving "up and down". Does light? If a "wave" of light does follow the analogy, how is it light is made up of photons and not made up of atoms of whatever medium the light is traveling through?

MarkF
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Light is a wave?

Quote:
how is it light is made up of photons and not made up of atoms


If light depended on atoms to propagate it would not be able travel through avacuum.
The behavior of light in eletro-magnetic theory is described without any propogating media for the wave to wiggle in. You can add a media but it will not improve the predictive power of the theory. The electro-magnetic field is normally regarded thing in and of itself as are electron fields, gravity fields etc.

KWSN-GMC-Peeper of the Castle Anthrax
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RE: RE: how is it light

Message 15965 in response to message 15964

Quote:
Quote:
how is it light is made up of photons and not made up of atoms

If light depended on atoms to propagate it would not be able travel through avacuum.
The behavior of light in eletro-magnetic theory is described without any propogating media for the wave to wiggle in. You can add a media but it will not improve the predictive power of the theory. The electro-magnetic field is normally regarded thing in and of itself as are electron fields, gravity fields etc.

What was that thing I read years ago... On Mondays, Wednesdays and Fridays light is a wave. On Tuesdays, Thursdays and Saturdays it's a particle. On Sundays, we pray. :D

Stick
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RE: What was that thing I

Message 15966 in response to message 15965

Quote:
What was that thing I read years ago... On Mondays, Wednesdays and Fridays light is a wave. On Tuesdays, Thursdays and Saturdays it's a particle. On Sundays, we pray. :D

Of course there was that other debate from years ago regarding the duality of lite: tastes great/less filling. ;-)

Mike Hewson
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RE: in physics class

Quote:
in physics class .........whatever medium the light is traveling Through?

You are in great historical company with these questions. I think the best reply is that light just is what it is, and it is hard to accurately analogize to anything else. In practical terms one can choose the 'wave model', the 'particle model', the 'Mexican wave theory', the 'weekday/weekend model' or whatever. If it suits your purpose it's trez cool.

Try this for size..... if we weren't all born so macroscopically LARGE, but grew up from Angstrom size, we'd be saying things like 'they don't make light like they used to in the old days' and 'what are those young people on about nowadays, can't they see what we are talking about?' :-)

For that matter, what do the LIGO people use? QED or what?

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Es99
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RE: in physics class we

Quote:

in physics class we learned that a wave is just individual atoms moving up and down. it was explained to me as similar to doing the wave at some sports venue. The only motion is individual ppl standing up and sitting down. The observed motion is a left to right movement.

sound waves hold true to this, they are air molecules moving "up and down". Does light? If a "wave" of light does follow the analogy, how is it light is made up of photons and not made up of atoms of whatever medium the light is traveling through?

Light does not need a medium to travel through. It is a constantly changing electric field and magnetic field that I think of as like someone pulling themselves up by their bootstraps. The changing electric field causes a changing magnetic field which causes a changing electric field and so on.. (someone stop me if I've remembered this wrong!)

The real question I think is "What is a wave?".

Physics is for gurls!

MarkF
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In Feynman's great little

In Feynman's great little book QED: The Strange Theory of Light and Matter he insists on calling everything particles light, electrons you name it. He hardly even mentions waves.

Odysseus
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RE: in physics class we

Quote:

in physics class we learned that a wave is just individual atoms moving up and down. it was explained to me as similar to doing the wave at some sports venue. The only motion is individual ppl standing up and sitting down. The observed motion is a left to right movement.

sound waves hold true to this, they are air molecules moving "up and down". Does light? If a "wave" of light does follow the analogy, how is it light is made up of photons and not made up of atoms of whatever medium the light is traveling through?

Unlike water waves, sound waves oscillate 'back and forth' rather than 'up and down' -- or 'side to side'. Technically speaking they're "longitudinal" while EMR, including light, is "transverse". At any instant a sound wave consists of alternating regions of high and low pressure in the medium; a better analogy than the standing & sitting crowd might be the traffic on a moderately congested freeway, where the widely spaced vehicles move quickly and the closely packed ones move slowly.

EMR doesn't seem to require a medium; although the idea of a "luminiferous aether" was popular for a while (and still has a few adherents), the results of the Michaelson-Morley experiment, first performed well over a century ago, are interpreted by most physicists as disproving the notion. Light waves consist of transverse electric and magnetic waves at right angles to each other, hence the description "electromagnetic".

Photons are a different kettle of fish: according to quantum theory, whether light acts like a particle or like a wave depends on the experiments you do to observe it. Both models are only analogies that break down under certain conditions, and they're not compatible with each other. This is related to Heisenberg's uncertainty principle; a wave has a precise energy but can't be localized to a point in space, while bouncing a particle off something can tell you exactly where it is but randomizes its energy.

Mike Hewson
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RE: EMR doesn't seem to

Message 15971 in response to message 15970

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EMR doesn't seem to require a medium

It's certainly difficult to analogise in everyday terms. The closest light has as a 'medium' is in fact spacetime - a conjoint entity of what was previously separate notions of space and time. When those neutron stars collide say, that cosmic rumble in spacetime radiates out from that area as what is termed gravitational waves ( or radiation or gravitons ) - and this alters the path of the light/photons in spacetime, which is what LIGO etc is all about. When the light from the big bang decoupled from matter ( about 300,000 years post big-bang ), it streched along with the stretching of spacetime ( the expansion of the universe ). We now detect the effect of that stretching by the much lower frequency ( and longer wavelength ) of such electromagnetic radiation compared to when it was formed - the cosmic microwave background. When light enters a black hole it is following a spacetime path of pretty steep curvature ( compared to around here ) inwards, such that a distant observer seeing a object approaching the event horizon will see it gradually slow down then 'freeze' ( and fade ). When light is lensed by the curvature in spacetime induced by the huge mass of a galaxy it is following a lesser warping of spacetime. For gravitational field strengths near Earth, spacetime is altered far more for time measurements than for distance measurements - the Global Positioning System needs to account for this else the 'drift' results in tens of kilometers per day inaccuracies ( speed of light is about an Imperial foot per nanosecond ). While it's difficult to visualise the extra dimension(s) that is the full arena that light travels in, if you include the time factor in the 'warping' it is easier. Roughly speaking the 'faster' you travel in space, the 'slower' in time. If you're uncomfortable with the idea of light having any medium then - just let photons be the surveyors of the landscape of spactime, they will follow the terrain, possibly turning up 'late' and 'tired' ( if at all ).

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Ben Owen
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Folks, You might check out

Folks,

You might check out this thread for some discussion of how light can propagate without a medium. You don't need quantum mechanics to explain it, by the way, and historically it came first.

I'll say something about quantum mechanics here before too long. I've developed a spiel which I give as part of my first lecture. But since I just finished grading the very long final for that class, I'll need a bit to recharge my batteries.

Stay tuned,
Ben

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