I see lots of drawings illustrating amplitude, frequency, etc using a sine wave to represent an EM wave, but why are they drawn like this? What do the crests and troughs mean/what values do they represent? And what do these waves look like in real life?
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Here is how waves happen:
1. Move something away from where it wants to be
2. Let go, and the thing goes back to where it wants to be
3. The thing overshoots, and now it’s where it doesn’t want to be again, only this time it’s on the opposite side.
4. When it goes too far it eventually stops, comes back, and oversoots again
5. Rrepeat over and over again.
Note that the thing, whatever it is, doesn’t snap back instantaneously. It picks up speed, it’s moving at its fastest when it crosses the centre line (where it wants to be) and then once it overshoots it starts to slow down. Eventually it stops, just for an instant, then as it starts coming back it picks up speed again.
This thing could be a guitar string. Or a child’s swing in a park. Or an electrical field. As long as we have a force which is trying to return the thing to its home position.
Now what we want to do is see the shape of this movement. So I want you to imagine you have a roll of paper like one from a cash register, and a marker pen. And imagine you’re watching a video of a kids’ swing. You’re going to step through the video frame by frame. On the tape you’re going to put a dot to show the height of the swing above ground. And after every frame you’re going to advance the paper roll a little bit.
Those dots will form the wave drawing that you’re asking about. The motion of something moving back and forth like we described, if you also move it through space, will form a wave like that. Guitar string, swing, electrical field. Doesn’t matter.
The force which makes a guitar string return to the centre is tension in the string. The force which makes the swing return to the bottom is gravity. (I’m using the word force informally here.) When we create an imbalance in an electrical field it wants to retun to its rest state. But what makes it overshoot? What makes it springy?
When the electrical field is busily returning to its rest state, tgat has a sideways effect of creating an imbalance in the magnetic field, which wants to get back to *its* rest state. And when the magnetic field is busily returning to its rest state, that creates a new imbalance in the electrical field. And so on, and so on.
The strength of the electrical field grows and reverses and shrinks and grows and reverses and shrinks, with the same pattern as the guitar string or the swing. That very same wave pattern.
And that is why we draw them as we do.
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