Light that heats

RadiationThere are three types of radiation: firstly the visible variety, secondly the invisible kind and finally actinic radiation. Infra-red is radiation of the second kind, noticeable by its heating effect. All three types share four characteristics. To begin with they can be transmitted through space; secondly they can be refracted; thirdly they can be reflected and finally they are obstructed by a black surface. This last explains — for example — why the photographic plate is a ‘negative’.