IGCSE
Physics Notes: Black and Silver Bodies – Reflection and Absorption
of 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 them. 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.
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.
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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