How Is Radiation Explained In Physics?

Both conduction and convection require a material medium from conveying heat from one part to another. Radiation, on the other hand, does not require any medium. The earth receives radiant energy from the sun in the form of electromagnetic waves which can pass through vacuum.

All bodies are continuously emitting and absorbing radiant energy. If a body emits more energy than it absorbs, its temperature falls. On the other hand, if a body absorbs more energy than it emits, its temperature rises. A body at a constant temperature, then, absorbs as much energy as it emits.

The rate at which the body emits or absorbs radiant energy depends on its temperature and the nature and area of its surface. A rough surface is a better absorber than a smoother one, because microscopically a rough surface has a more surface area.

Good absorbers are good emitters and poor absorbers are poor emitters.

If we pour hot coffee simultaneously in two metal cups of the same size and shape, but with one having a rough black surface and the other a bright polished surface, the coffee will cool faster in the black cup because the rough black surface is a better radiator. Thus coffee or tea remains hot in a shinning bright cup for longer. When iced water is poured in these empty cups, the water in the black cup will warm up faster since black is also a better absorber of radiant energy.

The base of an electric iron is highly polished so that it does not lose heat by radiation. Houses which are white washed or painted in light colours keep cooler in summer, because light surfaces do not absorb much solar radiation.