Infra-red is a kind of light that your eyes cannot see, but your skin can feel as warmth. The sun sends a lot of it; that is a big part of why a wall stays hot after the bright light looks gone. Heat lamps, some TV remotes, and night cameras work with it. Though actually, saying it ‘gives out heat’ is a little tidy: the infra-red radiation is the heat we feel, not a red glow plus heat on the side. It sits just past the red end of the rainbow — infra means ‘below’ — so it is not a colour of red you have failed to notice. British spelling often keeps the hyphen (infra-red); American spelling usually writes infrared as one word.