What is disturbed by a light wave

Zombrex

Apologies if this is a really simple (or dumb) question. I am working my way though A Brief History of Time and was reminded of a point that has always wondered about.

Light (and all electromagnetic energy) seems to behave like a wave (and also a particle in some experiments, but leave that aside for a minute)

Hawkins described these waves as disturbances, as one would. A sound wave is a disturbance of air. Sonar is a disturbance of water etc

My question is what does the energy from light disturb? What the does light energy travel through? Do we know?

Red Hand

There are those airless, light-bulb shaped tubes which have a mettallic material which shakes or twirls a little when an intense beam of light hits it.( Or did I just imagine or dream that in science class? I probably did dream it....:))

Its an interesting question...maybe light being an electro-magnetic wave disturbs a background magnetic field?

Son Goku

Apologies if this is a really simple (or dumb) question.

This question is probably one of the single greatest questions in all of physics. It basically lead Einstein on the road to relativity.

Wicknight wrote:

Hawkins described these waves as disturbances, as one would. A sound wave is a disturbance of air. Sonar is a disturbance of water etc

My question is what does the energy from light disturb? What the does light energy travel through? Do we know?

Maxwell's Equations were created by Maxwell in order to describe electromagnetism. Basically the equations say given a certain arrangement of currents and charges, you get this electric field and that magnetic field.
However, when playing around with the equations after a going for a walk, Maxwell thought he'd test his equations for inconsistencies. He checked the scenerio where there were no charges and no currents. He expected to see no electric and magnetic field, since there was no currents/charges to cause them.

What he found was that it was still possible to have electric and magnetic fields around. The fields behave like waves and look like this:

Based on the speed of the waves, Maxwell guessed correctly that these wavey electric and magnetic fields were describing light.
(He also used the solutions to predict the existence of radio waves, a well verified prediction if ever there was one).

Light is basically a bunch of self-contained electric and magnetic fields, turning themselves on and off over and over again as they travel through space. It isn't really a disturbance in anything, it just obeys the same wave-equations disturbances in water do.
i.e. The fields are just acting like waves

On a side note when Maxwell checked the speed of the waves it turned out to be the product of two fundamental constants and since the fundamental constants are the same to all observers, it implied that the speed of light was the same to all observers, which lead to relativity.

Zombrex

Son Goku wrote:

This question is probably one of the single greatest questions in all of physics. It basically lead Einstein on the road to relativity.

LOL .. Einstein has nothing on me

[EDIT]
Just read on Wikipedia that Maxwell himself believed that there must be a medium for the waves (the luminiferous aether), and that Einstein showed this wasn't necessary. This is all fascinating stuff.

Son Goku wrote:

Light is basically a bunch of self-contained electric and magnetic fields, turning themselves on and off over and over again as they travel through space. It isn't really a disturbance in anything, it just obeys the same wave-equations disturbances in water do.
i.e. The fields are just acting like waves

So its not really a "wave" through something (like a sound wave through air) but it is an electric field around a particle that oscillates similar to how a wave changes pressure? Or does it even have to be around a physical particle, can the fields exist just on their own?

If I have that right that certainly makes more sense to me.

Does that mean that there isn't a conflict between particle theory and wave theory, since the particle of light is generating this field around itself, and as such it can effect things around it like it was a wave, not just simply what is in front that it is going to hit?

Son Goku

Wicknight wrote:

Or does it even have to be around a physical particle, can the fields exist just on their own?

The fields can exist on their own. The Electric and Magnetic fields can exist independantly of any matter at all.
Physical particles like the electron do create electric and magnetic fields, but the fields can still exist independantly.

Does that mean that there isn't a conflict between particle theory and wave theory, since the particle of light is generating this field around itself, and as such it can effect things around it like it was a wave, not just simply what is in front that it is going to hit?

The photon, the particle of light, is what the fields are made out of. In classical mechanics, the electric and magnetic fields are the fundamental entities and light is simply an oscillating collection of these fields.
In Quantum Mechanics however, it turns out the fields come in little lumps, these being the photons.

Unfortunatly there is two issues here that kind of make things a bit confusing, but I'll explain as best I can.

The best analogy I can think of is water. The atoms and molecules of the water behave like particles and waves due to quantum mechanics. However get enough water molecules and you can make a river, which has waves of its own (e.g. the waves when you throw a rock in) that aren't related to the wave-particle duality of the molecules it's made out of.

Similarly photons have wave/particle duality. Get enough photons together and you can make a electric and magnetic field, which is able to have waves of its own (light).

Classical electric and magnetic fields, can behave in a variety of ways, which gives rise to all the electromagentic phenomena you see around you. When these fields behave like a wave they are called light.
The classical electric and magnetic fields are then themselves made of photon particles which have wave/particle duality like any other particle.

I hope I explained it decently enough. If there's any questions, just ask away.

Zombrex

Ok not going to pretend I understood all that but its a good start, and certainly cleared up a few stumbling blocks for me

I suppose the hardest part of modern physics for me is thinking about something that actually exists but that isn't actually made of anything, like an electronic field. ie something that has no matter.

I'm used to viewing all interactions in the universe as interactions between two physical particles. My pencil moves because my finger bangs into it and pushes it. A comet cracks in two because a meteorite slams into it at high speed.

The idea that something with no mass, such as an electric field, can just exist is hard to visualise, likewise the idea that something can interact with something else without two physical things "touching" or banging into each other.

Of course I realise that when we (or anything) actually touch something it is not our atoms banging into other atoms that move my pencil on my desk, it is the forces between the atoms. None of the atoms in my body, or the pencil, are actually "touching" each other.

Its difficult to get ones head around when one is used to viewing these things in terms of objects in the world. But I'm getting there

ZorbaTehZ

Very interesting thread tbh.
Nice one guys.

planck2

Light or electromagnetic radiation in general interacts with electrically charged particles/matter.

The interaction of light and matter is best described by a physical theory known as quantum electrodynamics perfected in the early 1950's. Feynman, schwinger and Tomonga received the 1965 Nobel prize in physics for their work in this area.

I suggest you read Feynman's book on the subject entitled "The Strange Theory of Light and Matter"

mawk

Jeremiah 16:1 wrote:

There are those airless, light-bulb shaped tubes which have a mettallic material which shakes or twirls a little when an intense beam of light hits it.( Or did I just imagine or dream that in science class? I probably did dream it....:))

Its an interesting question...maybe light being an electro-magnetic wave disturbs a background magnetic field?

I think you mean a crookes radiometer

http://upload.wikimedia.org/wikipedia/en/thumb/1/1d/Crookes_radiometer.jpg/180px-Crookes_radiometer.jpg

i kinda want one.. think itd be a good ornament..

JoeB-

Wicknight wrote:

Does that mean that there isn't a conflict between particle theory and wave theory, since the particle of light is generating this field around itself, and as such it can effect things around it like it was a wave, not just simply what is in front that it is going to hit?

Hmmm... I'm not an expert but when people say that light behaves like a wave they mean that it shows some effects that waves like water waves do... i.e interference and diffraction... (I think it's diffraction... when waves tend to go around corners or expand into the area behind an obstacle... a straight line particle wouldn't be able to get into the area behind a obstacle but light 'waves' can...)

Interference is when two sources of light combine to produce 'highs' and 'lows'.. i.e areas where waves overlap and combine to form larger waves (larger amplitude) and other areas where the waves cancel each other out, this isn't easily explained by considering light as a particle.


However when light strikes a particle it appears to act as a particle itself, i.e it can knock electrons into a higher orbit and the whole light wave/particle is then gone... ??? There are probably better situations where the particle nature of light is clearer...

mawk

JoeBallantine wrote:

Hmmm... I'm not an expert but when people say that light behaves like a wave they mean that it shows some effects that waves like water waves do... i.e interference and diffraction... (I think it's diffraction... when waves tend to go around corners or expand into the area behind an obstacle... a straight line particle wouldn't be able to get into the area behind a obstacle but light 'waves' can...)

Interference is when two sources of light combine to produce 'highs' and 'lows'.. i.e areas where waves overlap and combine to form larger waves (larger amplitude) and other areas where the waves cancel each other out, this isn't easily explained by considering light as a particle.

i think your getting at rarefaction..

anyway there is a very easy way of proving that light is a wave form, polarisation. take two pairs of polarised sunglasses and hold them one in front of the other, then simply turn one pair 90 degrees and look through the two lenses.. all the light vanishes. because light acts as a transverse wave.

anyhow a wave doesnt have to disturb anything. which is why light can move in a vaccum. now im not a lawyer but if i remember right : A wave is a disturbance that propagates, carrying energy. Apart from electromagnetic radiation, and probably gravitational radiation, which can travel through vacuum, waves exist in a medium (which on deformation is capable of producing elastic restoring forces) through which they travel and can transfer energy from one place to another without any of the particles of the medium being displaced permanently; i.e. there is no associated mass transport.

actually i was too lazy to type that so i c/p'ed it.. but anyhow as you can get, light is a special case

ArthurDent

planck2 wrote:

Light or electromagnetic radiation in general interacts with electrically charged particles/matter.

The interaction of light and matter is best described by a physical theory known as quantum electrodynamics perfected in the early 1950's. Feynman, schwinger and Tomonga received the 1965 Nobel prize in physics for their work in this area.

I suggest you read Feynman's book on the subject entitled "The Strange Theory of Light and Matter"

Any of Fenymans books are a very good place to start for anyone interested in science

Kevster

planck2 wrote:

Light or electromagnetic radiation in general interacts with electrically charged particles/matter.

Exactly, and when matter absorbs light, the bonds between the atoms that make-up that matter begin to vibrate quicker, right?

planck2

Kevster wrote:

Exactly, and when matter absorbs light, the bonds between the atoms that make-up that matter begin to vibrate quicker, right?

my intuition says the molecules have a polarisation tensor, loosely speaking this is a matrix, and that this polarisation tensor will react to the polarisation of the electromagnetic wave.

i think that if a bond has a polarisation vector and if we place this bond in an electromagnetic field which has a polarisation in a certain direction then the bond will begin to oscillate only if the bond's polarisation has a component in the direction of the fields.

this is a classical answer to this question.

if an atom however is impinged upon by an electromagnetic wave then the atom may or may not go into an excited state. whether it does so depends on the frequency of the em wave and the energy gap between the energy levels in the atom. if these are equal then the atom becomes excited. this is followed by the emission of a photon (a particle of the em field).

for a "slab" of matter quantum mechanically speaking light will interact with the matter in 3 different ways in general. these are scattering, the photoelectric effect and pair production. the last absorb the photons from the impacting em field, the first one scatters the beam.

in general the molecules will vibrate and break apart only if the frequency of the impinging em wave is high enough.

Michael Collins

planck2 wrote:

...if an atom however is impinged upon by an electromagnetic wave then the atom may or may not go into an excited state. whether it does so depends on the frequency of the em wave and the energy gap between the energy levels in the atom. if these are equal then the atom becomes excited. this is followed by the emission of a photon (a particle of the em field).

Aye. This pretty much how Light Emmiting Diodes (LEDs) work, except for the electrons are excited up past the energy gap by the applied voltage as opposed to an impinging wave (although that will do it too). Then they drop back down and emit a photon with the colour according to Plank's equation...

Kevster

Damn - yes - I forgot to take-into account the differences in frequency between the bond oscillation(s) and that of the em wave.



"for a "slab" of matter quantum mechanically speaking light will interact with the matter in 3 different ways in general. these are scattering, the photoelectric effect and pair production. the last absorb the photons from the impacting em field, the first one scatters the beam."


Can you give me a practical example of something that scatters the beam? - A prism, no? Also, what is the Photoelectric effect in relation to this.

planck2

Kevster wrote:

Damn - yes - I forgot to take-into account the differences in frequency between the bond oscillation(s) and that of the em wave.



"for a "slab" of matter quantum mechanically speaking light will interact with the matter in 3 different ways in general. these are scattering, the photoelectric effect and pair production. the last absorb the photons from the impacting em field, the first one scatters the beam."


Can you give me a practical example of something that scatters the beam? - A prism, no? Also, what is the Photoelectric effect in relation to this.

Not at this time I am afraid. I am too busy writing a term paper. If no one has gotten back to you in about 2 weeks I will reply then. The photoelectric effect is very important. look it up on wikipedia.

Kevster

Hey, don't worry about it. I am immensely busy too: Finishing up one course and going straight into the final year of another course next year. I'll look it up on the WWW when I get time.


Take care,
Kevin.