Heat transfer

Radiation is the transfer of thermal energy by rays (electromagnetic waves); it passes even through a vacuum, which is why the Sun’s energy reaches the Earth across empty space. Every heated body emits rays — the higher the temperature, the stronger the radiation. A body can absorb or reflect radiation: dark, rough surfaces absorb and emit well, while light-coloured and shiny surfaces reflect more. So light clothing is advised in summer, buildings are painted white and fuel tankers are given a silvery coating; solar collectors, on the other hand, are black. A thermos reduces all three kinds of heat transfer: the air is pumped out between its two walls (a vacuum), so there is no conduction or convection; the inner surfaces are mirror-bright, so radiation is low; and the stopper conducts heat poorly. Convection: steam boiler, ventilation; conduction: cooking pots; radiation: greenhouses, solar water heaters.