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Grand-Tension8668 OP t1_j8k7co9 wrote

Reply to comment by Movpasd in When measuring the wavelength of EM radiation... what's actually being measured? by Grand-Tension8668

Thanks a lot for this reply. I've definitely started recognizing what you're trying to say in those other posts, that things really approach a point where you need to trust the math and coming at it the other way around fails to create an accurate understanding of things. (And that our intuitive understanding of what "stuff" is doesn't really hold water in an absolute sense).

I think I'm coming out of this with a less incorrect "mode C" mental model, at least– EM fields change over time / distance (one in the same in this case but whatever) in a cyclical way, so they're waves. We can measure how long it takes for one "wave cycle" to happen, as in the distance traveled as a point oscillates between the electo- part and the -magnetism part. ...Which is certainly still a pretty wrongheaded explanation and I really need to start learning the math of physics in my spare time.

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aiusepsi t1_j8miaub wrote

EM fields in general aren't cyclical (I would usually say 'periodic' for what you mean by that), for example consider the magnetic field surrounding a magnet; that's completely static, so it doesn't change over time, and it just gets weaker with distance from the magnet.

Electromagnetic waves are, mathematically, a lot like lots of other kinds of waves, like sound waves, or waves on the surface of water. All your need is some kind of field, that is, a property which exists at each point, and for the physics of that field to obey a particular form of equation, and waves will exist in that field. For obvious reasons, that kind of equation is called a 'wave equation'.

For sound waves, the field is air pressure. On the surface of water, it's the height of the water's surface. Those are scalar fields, that is, those properties can be described by a single number. EM is a bit different because it's a vector field with two vectors at each point in space. But ultimately, they end up looking very similar.

If you graph out air pressure on one axis and space on the other axis for a sound wave, you get a sinusoid, i.e. it looks the same as graphing y = sin x. And that's exactly what you get if you graph out just the length (i.e. magnitude) of the electric field vector (rather than worrying about the direction of the vector) along the direction the wave is travelling. Just pretend it's a scalar field like air pressure!

Graphing out the magnitude of the magnetic field is basically the same, the electric and magnetic fields vary in the same way in an EM wave, just that the electric and magnetic field vectors are at right angles to each other, and they're both also at right-angles to the direction the wave is travelling.

Anyway, for all of the above, wavelength is just the distance between adjacent peaks on the graph.

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Grand-Tension8668 OP t1_j8morhq wrote

...Well, it's helpful to be reminded that EM fields aren't necessarily changing their properties over time, at least

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Movpasd t1_j8qvfax wrote

I'm very glad you found my reply useful!

If you're set on continuing to teach yourself physics (which I think is a very good, though time-consuming idea), I'd start by making sure you're on top of your high school/A-Level maths and physics (KhanAcademy is a great place for this), and then move onto some first-year university introductory textbooks. You don't have to read them back to front -- start with the first chapter, take your time, do the exercises, and when you get bored switch to a different book. (I really like Griffiths' textbooks, but YMMV.)

A good search term is "introduction to [topic]" or "introductory [topic] textbooks". Good topics to start with would be classical mechanics, electrodynamics, and quantum mechanics. You could then move onto special relativity and statistical physics (my favourite!).

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