Mark H's reply gave me a sort of epiphany. Suddenly things started to fit together.
My undergrad physics class presented light as waves. Light can be polarized, linear or circular or elliptical. Or it can be unpolarized, a random mixture of the others. It has frequency and wavelength and constant velocity in vacuum. I didn't understand what would make light be like that. Why is that the fundamental nature of it? What's really going on?
Now I see. Classically, EM radiation is emitted by an accelerating charge.
Say you have a charge $S$ and a charge $T$. Call the vector between them $n$. $S$ has a velocity $v$ in the same direction as $n$, and also an acceleration perpendicular to $n$ and $v$. Then there is a force on $T$ that is perpendicular to $n$, that IS the radiation. This situation -- $n$ and $v$ same direction, acceleration perpendicular -- creates the strongest radiation.
If the source happens to oscillate back and force in a sine wave, then it will create a linearly-polarized wave of radiation perpendicular to its motion.
If the source happens to travel in a circle at constant angular velocity, then it creates radiation that is linearly polarized in the plane of its travel, and circularly polarized perpendicular to that plane, and elliptically polarized in between.
If the source follows some other path its motion can be split up into sine waves with fourier analysis, so it will look like the same thing divided into pieces.
The only reasons to think of light as polarized sine waves are first that charges do often oscillate like sine waves or in circles (or ellipses). And second that fourier analysis makes it convenient to think of them that way regardless.
So if a charge goes through a linear accelerator, being accelerated the whole time, it makes radiation. The radiation is strongest perpendicular to the particle. There is no wavelength. There is no frequency. It's just a pulse of radiation that lasts from the time the charge is first accelerated until it stops being accelerated. When the radiation encounters a charge it pushes the charge sideways compared to the direction from the source charge at the time the radiation left it, to the target charge at the time the radiation arrives.
There doesn't have to be any wave. Just, sometimes there is.