Types of light sources
Light from hot materials
A flame gives out heat and light – e.g. a match or a candle. However it is not necessary for things to be burning to emit light. If a piece of metal is heated to a high temperature it gets red hot (600°C upwards) or maybe white hot (above 3000°C) as in the case of an electric light. (In an electric light the glass bulb keeps air away from the hot metal filament and so it does not burn.)
We get light from the sun by a similar process; the visible surface of the sun is very hot (5780°C) and so it emits heat and light.
The operation of fluorescent lamps (and CFLs) is complicated. In these the light is emitted by a white powder on the inside of the glass; the powder does not have to be hot but it must be irradiated by ultra violet light. Fluorescent marks are printed on all euro notes for added security; these are easily seen in ultra violet light.
‘Brighteners’ that are added to some washing powders work on the same principle; they make clothes appear ‘whiter than white’ under ultra violet lamps. Luminous paints, some minerals, wing markings on some butterflies and a few flowers fluoresce in the same way.
Fluorescent materials do not produce light on their own but only when they have been exposed to ultra-violet light!
A number of chemical reactions produce light (without noticeable heat) and a number of living things make use of this technique, e.g. fireflies, some fungi, many deep-sea fish and other marine animals. So-called ‘glow sticks’ operate in the same way.
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