Ummm ok, so what is oscillating exactly in an electromagnetic wave?

I know very little about EM waves (which will soon become apparent) so be gentle, but, more importantly, please don’t use that weird sideways Neptune’s trident symbol in any attempts at explanation.

So. What variables are changing as the wave goes from peak to trough in an EM wave? What is the difference to me (or anything) if a photon strikes my retina at the peak vs. the trough vs. Y=0? I was reading about the double slit experiment and since there are light and dark patterns depending on whether there is constructive or destructive interference, does that mean that if a photon hits my retina where Y=0 that I don’t see any light? I’m sure I’m confusing the wave and particle-like natures of light but don’t we all really?

In regards to EM radiation hitting your retina, you must keep in mind that light acts as a wave and a particle, and in this instance, it’s the photon that’s the particle in question. A photon is emitted or absorbed by the electrons that exists in the light source and in your retina, respectively. It’s this quantum packet of EM energy that’s more relevant when it comes to what your retina is “seeing”.

As to what’s “oscillating” I’ll leave for physicists here to answer, but as far as I know, it’s a question that’s best explained using Relativity.

ETA: I should say, however, that the wavelength of light is important when it comes to what can enter through your pupil.

Just a comment for the second part of your question: one photon won’t do much to create anything perceivable. There needs to be a minimum, and rods are more sensitive (in this respect) than cones and can trigger with fewer. And they are sensitive to different wavelength ranges. Realistically though, many, many photons will hit your retina.

For the rest, I don’t think many of our sensory organs interpret the stimulus at one small slice of time, but instead look at the changes over a larger period of time (which is incredibly short to our perception). So the position on the y axis at a single x position isn’t really an issue.

Interference requires two waves (except the double slit experiment shows it sort of doesn’t) and a photon is the particle nature of light, so talking about destructive interference for a single photon is doubly meaningless.

Oh, and what’s oscillating is the electromagnetic field. I wouldn’t try making sense of it if I were you. And I say that because I’ve given up myself. :wink:


The simplest way to think of it is in terms of electrically charged test particles (i.e. particles which will be affected by physical phenomena, but whose physical properties are considered too negligible to affect anything else). The wavelength will be the distance at a given instant of time between (the nearest possible) two such test particles who feel the maximum force due to the electric field of the em wave and when that force is in the same direction. The force felt by the test particle will always necessarily be at right angles to the direction of propagation of the em wave. Note I’m ignoring the magnetic field for simplicity’s sake, but the relationship between the electric and magnetic fields of an em wave is very simple.

If we take a linearly polarized em wave, then: at a crest the force on the test particle is at a maximum in one direction which is at right angles to the direction of propagation; at a trough the force is at also at a maximum, but in the opposite direction; and halfway between a crest and a trough the test particle feels no force at all. Note though that the positions of crests, troughs and halfway points change with time.

When visible light strikes your retina it causes particles within to oscillate as they are successively subject to crests and troughs. However the particles in your retina cannot be thought of as test particles because their effect, at this level is not negligible so their own em fields cannot be neglected and the em field of the light wave will be affected.

In the double-slit experiment the dark bands are due to the wavelets generated by the slits always destructively interfering, independently of time, at certain points. Or in other words at these points the force due to the electric field of the wave on a test particle will always be zero, independently of time.

If you were to invoke the particle-like nature of light (by for example firing photons one at at time through the double slits or asking what happens at the level of a single photon interacting with a particle in your retina) you necessarily invoke quantum physics and the description will be more complicated.

A photon then is not necessarily a single wave in the sense you describe and interference does occur in quantum physics in situations when only a single particle is involved.

The question of “what is oscillating” stumped many of the brightest minds for a long time. Sound waves are really just the motion of air molecules. Water waves are something that takes place in the water medium. So it made sense that EM waves must be something “waving.” But it must be really really thin and hard to detect, because we can move through it with pretty much no friction, and we can see distant stars through it.

The Michelson-Morley Experiment was intended to detect the Earth’s relative motion through this medium, which they called the aether. It was shocking to them to find that there was nothing there.

That leaves us with the idea that there are waves, but nothing that’s waving other than the electric and magnetic fields. But what are fields, really? It’s just a model we made to describe the way things work, mathematically. It doesn’t have an intuitive explanation, but does it need to?

At the bottom, physics is simply our models of how the world behaves. We have developed the model for electromagnetism so that it’s useful and accurate, within the limits of when it applies. They describe what’s going on in reality, but it doesn’t necessarily have to jive with your intuition. Your intuition evolved to handle things on a scale that we deal with every day, and it just doesn’t work that well to handle the very small, and warped space. Sorry.

Thanks Asymptotically Fat, that was helpful. Somehow I’m guessing that the force isn’t real in a sense because photons aren’t attracting other fundamental particles with this force, yes?

This makes me feel better.

Sucks being stuck in Dawkin’s Medium-Sized World. :wink:

A changing magnetic field generates an electric field. That’s why moving a magnet near a wire generates an electric field inside the wire, which causes electricity to flow. This is how a generator works.

Also changing electric field generates a magnetic field. That’s how an electromagnet works.

Now, a changing magnetic field generates a changing electric field not just inside a wire, but in empty space as well. And that changing electric field generates a changing magnetic field ahead of it, again in empty space. And so on, repeatedly. So this is now a packet of energy travelling through empty space.

(That may not be rigorously accurate, but that’s how I visualize Maxwell’s equations which describe how an electromagnetic wave behaves.)

Here’s an animated .gif file that might help (there’s a link to the original YouTube video as well). In that image, the EM wave is traveling in the +X direction. The electric field amplitude is shown by the red vectors, and the magnetic field amplitude is shown by the blue vectors.

Note that the red and blue vectors don’t have a spatial size. Their length is field amplitude, not spatial length.

The force is real, however what I’ve considered really is the force exerted by the em wave on test particles that are held motionless by some independent correcting force (so as to ignore the effect of the magnetic field), which is obviously highly unrealistic in regards to how actual particles behave. It was mainly to give some sort of physical meaning to the oscillations of em waves. But your right in that this certainly not a good simplification for how real particles behave in the presence of em waves.

On the level of an individual photon and fundamental particles the behaviour is fundamentally quantum.

As you note, the key word is "perceivable’. This is one of many cites/sites to confirm that the retina is sensitive to single photons (although perceiving them may be a different matter).

1.A changing magnetic field creates an electrical field.
2.A changing electrical field creates a magnetic field.

See the electrical field that got created in 1 ? create ? change !.
So the electrical field in 1 is the changing electrical field in 2 , which create a magnetic field… a changing magnetic field , as in 1…

Its circular… It goes on for ever.

The speed of light in a medium is related to the properties of that medium that affect the energy of electric and magnetic fields.

μ for the vacuum permeability or magnetic constant, ε for the vacuum permittivity or electric constant, speed c = 1/√(ε μ)

Yes, which is what I tried to imply without going into with the parenthetical about the double slit experiment, but a single photon will be detected somewhere as a single interaction, so the OPs question about a photon hitting your retina with amplitute 0 doesn’t make much sense, even if interpreting the question in the most positive way.

Yes and no. A single photon will be detected in a particular place as a single interaction only if you set out to so detect it. If you don’t, then the photon will appear to be following all possible paths simultaneously – i.e.- it will be acting as a wave. This is why the double slit experiment produces an interference pattern even when light is emitted one photon at a time.

Could it be a function of charges attracting and repelling, like in a capacitor?

Or is the OP asking about the medium for carrying this effect?

Actually, I think the human retina is capable of detecting single photons under the right conditions (although they are conditions that are unlikely ever to arise outside a laboratory).

In the context of talking about rods, as you were, that is pretty misleading. Rods are all the same and, unlike cones, don’t care much about frequency (so long as it is within the visible spectrum that they are sensitive to).

That, though, is true. I think all our sense organs, and certainly the eyes, are concerned with changes in stimuli over time rather than intensity at any instant.

See KarlGauss’ (accurate) interpretation the meaning.

Sorry, meant photoreceptors in general, yes. But rods are certainly responsive to light with a peak of ~500nm. It’s just that that doesn’t translate to a blue-green percept. But a photon outside of the “acceptable” range isn’t going to be interpreted like a different wavelength. I mean, doesn’t a red light just not affect S photopsin (or not much), rather than something in the cone itself or further on that ignores the “bad” stimulus?