What is the origin of the term 'red hot'?

Answer 1

Any substance heated to a high enough temperature will begin to glow, as the electrons in its atoms are knocked into higher orbits and then fall back (they release energy in the form of light as they fall back). The lowest energy electrons that produce visible light produce low frequency, long-wavelength light, and that's red. So when something is hot enough to glow, it first glows red, then yellow-orange, and finally white (as it begins to glow in all colors). That's why candle flames glow red, but welding torches glow white; big old cold stars like Betelgeuse are red but hot new ones like Procyon glow blue-white; light bulb filaments, if you turn the power way down, glow red (because they don't get enough energy to glow white).

So if you are heating something up in a flame or with electric current, "red hot" is the point where it's just starting to glow. (That's pretty hot!) Probably the place where people noticed it most in the old days was at the blacksmith's shop, where iron that was glowing red was hot enough (and therefore soft enough) to shape and bend with hammers and tongs on the anvil, or maybe among cowboys, where a red-hot iron was hot enough to leave a clean sterile burn that was less likely to get infected when you branded cattle.