radiation

Emission of radiant energy as particles or waves – for example, heat, light, alpha particles, and beta particles (see electromagnetic waves and radioactivity). See also atomic radiation.

All hot objects radiate heat. Radiated heat does not need a medium through which to travel (it can travel in a vacuum).

Most of the energy received on Earth arrives by radiation of heat energy from the Sun. Of the radiation given off by the Sun, only a tiny fraction of it, called insolation, reaches the Earth's surface; much of it (for example, radio waves, ultraviolet rays, and X-rays) is absorbed and scattered as it passes through the atmosphere. Visible light and infrared rays pass through the atmosphere, the infrared rays causing a rise in temperature. The radiation given off by the Earth itself is called ground radiation.

How much a surface radiates heat depends on its temperature and the type of surface. Dull black surfaces absorb more heat and therefore radiate more heat that polished shiny surfaces, which reflect heat and are therefore poor radiators of heat. For example, engine-cooling mantles in cars are black so that they radiate heat from the engine. A vacuum flask has a polished, silvery surface so as to keep hot liquids hot and cold liquids cold as no heat is radiated or absorbed.

The human body radiates heat at a rate of 100 joules every second. This is the same energy as radiated by a 100-watt light bulb.