radiation

Radiation is the transfer of heat as electromagnetic waves. Unlike the other two ways heat moves, it needs no material at all: it travels through a vacuum at the speed of light, which is how the Sun's energy crosses 93 million miles of empty space to reach us. Everything above absolute zero gives off some, but hot objects radiate far more.

It is the heat you feel without touching anything. The warmth on your face from a campfire, the heat of a heat lamp, and the sunburn you get on a cloudy day are all radiation arriving as waves. What happens next depends on the surface that absorbs it: dark, dull surfaces soak it up and warm quickly, while light, shiny surfaces reflect it away — which is why a white roof stays cooler and why a thermos is silvered on the inside.

On this subtest radiation is one of the three ways heat travels, and the questions ask you to sort them. Conduction passes heat by contact, particle to particle, as through a metal handle. Convection carries it by moving a warmed fluid, as in a boiling pot or a sea breeze. Radiation is the only one that crosses empty space, and the only one that needs no matter to carry it. Note that this sense of the word is about heat and light, not about nuclear radiation from unstable atoms.