Absorption and emission of radiation

The higher the temperature of the object, the higher the amount and more energetic the radiation it emits. A cool object still radiates energy but less than a hot object. Consider your hands as being warmed by the infrared radiation from a lamp for example. Though your hands also radiate energy, the lamp radiates energy to your hands at a much higher rate and thus your skin has a net gain in energy and an increase in temperature.

The appearance of a surface determines its rates of emission and absorption of radiation. In general, shiny coloured (e.g. white or silvery) and metallic surfaces emit or absorb radiation energy slowly since they reflect the radiation. Dark coloured (e.g. black) surfaces emit or absorb radiation energy more effectively. It is interesting to note that a good emitter of radiation is also a good absorber of radiation and vice versa.

Now you can see how people use different colours to enhance or reduce heat gain. People usually wear light-coloured clothing in hot summer to reduce radiation heat gain from the sun, and dark coloured clothing to increase heat gain in cold winter. Solar panels are usually black in colour to increase radiation gain. On the other hand, suits and instrument used for space travelling is usually silvery or white in colour. Do you know why?

 
Fig. 1-16 The surface of this balloon is silvery white in colour. This can help to reduce radiation energy absorption.   Fig. 1-17 The Hubble Space Telescope is covered by a layer of silvery material. Do you know why?

The following links to a toy called radiometer that makes use of the above principle.
The magical blades

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