Clashing Electromagnetic Waves

[ficklefiend]

waves move at different speeds according to the medium they are in .

Presumably, the diversity of the radio waves in the space above my head, earlier today, were all travelling through the same medium. And I thought that the speed of electromagnetic waves was a constant, through any specific medium?

Oh look what I found.



A model of an EM field moving through spacetime.

I think my knowledge gap lies somewhere in there. EM field bends spacetime, gravity bends spacetime, EM field is bent by gravity bending spacetime? Isn't this heading towards the unified theory thing? I dunno. Damned tricky stuff.

[Feck]

It only 'listens' for one frequency at a time though.

When waves in a pond interfere with one another, their original attributes/characteristics dissolve into another combined pattern. So, if those original attributes are destroyed by the combination event, then what remains of the original wave to 'listen' to?
I'm wondering why, for instance, the original radio waves that leave the BBC aren't lost in the interferences that they encounter before they enter my radio.

And they uncombine because they never actually combine in the first place. Imagine two waves on a pond, they never actually combine as one, they just pass through each other... but if their peaks and troughs occur in the same place, they become the sum of both.

I don't see how two material forces can pass through each other without affecting one another.

the particles in wave do not travel .. when a wave goes across an ocean the water does not, it just goes up and down....
Water waves do not stop each other they act independently what we see is the addition of 2 effects (if you use 2 wave generators)

[jamest]

Look at it the other way round and wonder why should they affect one another anyway?

Do you play snooker or pool?

[ficklefiend]

Ok, so we seem to have gotten this far - EM waves do interact, but in simple terms their pattern is not destroyed by other waves, their pattern is combined.

Next- how do they uncombine? Why can my radio still pick up two (or rather many) separate wave frequencies?

It only 'listens' for one frequency at a time though.

And they uncombine because they never actually combine in the first place. Imagine two waves on a pond, they never actually combine as one, they just pass through each other... but if their peaks and troughs occur in the same place, they become the sum of both.

So it is to do with their direction?

Of course, you couldn't send two waves in exactly the same direction at exactly the same time, could you? So that never happens and they are always just passing through each other.

Not the direction. Wave "interference" gives the wrong impression of what goes on really. Waves just do their own thing, but if their peaks and troughs ever happen to coincide, they add amplitudes. So a 5cm peak plus a 5cm peak becomes a 10cm peak, same with troughs. They may only coincide in a short area, after that they just go on their way.... or they may coincide again and again and again... then they keep adding amplitudes, like a sonic boom.

More their frequency then. At different frequencies they are less likely to remain in sync?

p.s- you say it is not about "direction" then speak about "going their own way". Not that it is your fault, but I think our metaphor based conveyance of ideas starts to crumble about here.

[Pappa]

Look at it the other way round and wonder why should they affect one another anyway?

Do you play snooker or pool?

Yup.

[jamest]

the particles in wave do not travel .. when a wave goes across an ocean the water does not, it just goes up and down....

Are you saying that 'light' just bobs up and down? Clearly not. This discussion was supposed to clear my confusion, not add to it.

Water waves do not stop each other they act independently what we see is the addition of 2 effects (if you use 2 wave generators)

I'm not thinking that there shouldn't be any definite radio signals. I'm just thinking that they should all be garbled - mixed, like white light. But clearly, they are not. Hence my desire for a clear explanation.

[FBM]

the particles in wave do not travel .. when a wave goes across an ocean the water does not, it just goes up and down....
Water waves do not stop each other they act independently what we see is the addition of 2 effects (if you use 2 wave generators)

Exactly. It's not stuff itself zipping through space, therefore no reason to expect things to bounce off each other. When em waves of comparable frequency are in phase, the result is contructive interference, when they're out of phase, it's destructive. Anywhere in between and they just keep going without interaction. That's my understanding, anyway.

[ficklefiend]

I think that all references to water waves only hinder understanding for lay people. Seems to be a common problem in the teaching of physics.

[Pappa]

I think that all references to water waves only hinder understanding for lay people. Seems to be a common problem in the teaching of physics.

Actually, I think they help a lot. They explain what waves do in a pretty clear way.

[ficklefiend]

I think that all references to water waves only hinder understanding for lay people. Seems to be a common problem in the teaching of physics.

Actually, I think they help a lot. They explain what waves do in a pretty clear way.

But they cause confusion when you try and lift that "common sense" knowledge into something less obvious, like EM waves. Sound waves and water waves move through gas and liquid, jumping to EM waves without making clear the difference in the "mediums" they travel in is confusing.

[Pappa]

I think that all references to water waves only hinder understanding for lay people. Seems to be a common problem in the teaching of physics.

Actually, I think they help a lot. They explain what waves do in a pretty clear way.

But they cause confusion when you try and lift that "common sense" knowledge into something less obvious, like EM waves. Sound waves and water waves move through gas and liquid, jumping to EM waves without making clear the difference in the "mediums" they travel in is confusing.

But the physics is the same.

[Xamonas Chegwé]

the particles in wave do not travel .. when a wave goes across an ocean the water does not, it just goes up and down....

Are you saying that 'light' just bobs up and down? Clearly not. This discussion was supposed to clear my confusion, not add to it.

Water waves do not stop each other they act independently what we see is the addition of 2 effects (if you use 2 wave generators)

I'm not thinking that there shouldn't be any definite radio signals. I'm just thinking that they should all be garbled - mixed, like white light. But clearly, they are not. Hence my desire for a clear explanation.

You never 'see' white light. You have three types of cones in your eye - Yellow, Green & Violet - they are each only triggered by photons of the corresponding frequency. When all three fire off together, your brain interprets this as 'white'. What you actually see is a mixture of all three colours, all travelling as separate waves and all passing through each other on the way to your eye - if they arrive in the same proportions as bright sunlight, it appears white.


Edit: Got my cones wrong - its YGV, not RGB, in the eye!

[ficklefiend]

I think that all references to water waves only hinder understanding for lay people. Seems to be a common problem in the teaching of physics.

Actually, I think they help a lot. They explain what waves do in a pretty clear way.

But they cause confusion when you try and lift that "common sense" knowledge into something less obvious, like EM waves. Sound waves and water waves move through gas and liquid, jumping to EM waves without making clear the difference in the "mediums" they travel in is confusing.

But the physics is the same.

I said it was confusing. I didn't say it was wrong.

[The Dagda]

These are the times I wish I understood physics better.

Although definitely radio waves are subject to interference... can you make head or tail of this wiki article?

http://en.wikipedia.org/wiki/Linear_superposition

I'm trying.



I don't even understand how they always travel at the same speed, surely the electromagnetic fields are not constant through space? Why is the speed constant then? Argh.

waves move at different speeds according to the medium they are in .

More precisely "photonic" waves always propogate at c in any medium, the interaction with the solid atomic structure of a mediums lattice though may result in a cumulative delay overall, hence diffraction. However photons never propogate at less than c. Something that only really is revealed post school level education. Incidentally if light were to propagate at less than c then Einstein's special relativity would be questionable at best.

Unfortunately the analogy of a water wave is imprecise as it deals with an atomic medium where actual matter is transferring energy, it would be better to visualise waves as energy distributions between cos x= 0 and cos (x) =1, which is just a way of saying that the wave is a half rotation of a circle followed by another half of rotation of a circle . When a wave passes another it may be unaffected overall, the amplitude of the wave may be boosted constructive interference or it may be cancelled out, or destructively interfered with. In a medium with billions of photons only when a significant amount are interfered with to any degree will interference become audibly apparent on your radio.

[FBM]

The speed of light (usually denoted c) is a physical constant. It is the fastest speed at which energy or information can travel, and is only attained by massless particles and waves such as electromagnetic radiation (e.g. radio waves, visible light, or gamma rays) in vacuum, where there are no atoms, molecules or other types of matter that can slow it down. Its value is exactly 299,792,458 metres per second,[1][2] often approximated as 300,000 kilometres per second or 186,000 miles per second (see the table on the right for values in other units)...The actual speed at which light propagates through transparent materials, such as glass or air, is less than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c / v). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 ≈ 200,000 km/s; the refractive index of air for visible light is about 1.0003, so the speed of light in air is very close to c.

http://en.wikipedia.org/wiki/Speed_of_light