Black and Silver Bodies - Reflection, Absorption and Radiation

You may know already that shiny things are shiny because they reflect all – or almost all – of the radiation that shines on them, and that a black thing is black because it absorbs all – nearly all the radiation that shines on it. In fact these properties of being able to reflect or absorb radiation extends beyond the light we can see into many other parts of the electromagnetic spectrum.

Black things left in the sun heat up more quickly because they are able to absorb more of the radiation shone on them. Shiny things take longer to heat up because they reflect most of the radiation that shines on them. It works the other way round. Black things cool down more quickly if left in the shade, and silver things cool more slowly because black things efficiently emit radiation and silver things emit radiation inefficiently.

We can use this in some surprising ways. For example if someone is suffering from hypothermia, it might seem wise to wrap them in something black, so they can get warm quickly. In fact this would mean they would radiate away their own body heat. Instead they are wrapped in aluminium foil because the human body is typically at a higher temperature than its surroundings so that the body heat which is radiated is relected back to them. Aluminium foil is found in every first aid box at sea for this reason.

Ever wondered why Arabs wear white? It is for the exact reason that they will absorb little of the heat that shines on them and keep the wearer cool.

The white clothes above keep the wearer cool. A patio heater has a silver hood to reflect down the heat from the heater which would otherwise be lost into space. In this way it keeps the patio

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