Comment by dTal
Although it's about wire transmission of photography - which, as pointed out, television is not - it's still well worth watching this 1937 newsreel explaining how it works, mostly because 1) they devote a LOT of time towards explaining the concept of scanning/rastering, which was clearly not widely intuited at the time, and 2) they do it with a brilliant physical analogy, with the incredible pedagogical clarity typical of such 1930s educational videos.
First time I watched that I forgot I had the speed set to 1.5 or something so the already fast-talking 1930s mid-atlantic radio announcer voice got exaggerated to a hilarious degree. Especially funny as the first few lines were about the importance of speed!