Libby Lowe of Henley Primary School asks :-

How come when you touch a light bulb it is hot?

John Campbell, a physicist at the University of Canterbury, responded.

One way to produce light is to heat something up until it glows. In light bulbs an electrical current is passed through a fine wire of the metal called tungsten which doesn't melt even when glowing white hot. You can see this wire in a torch bulb. It is protected from 'rusting' away by being surrounded by an unreactive gas contained in a glass bulb hence the name light bulb. This isn't a very efficient way of turning electrical energy into light. Only about a tenth of the electrical energy used is emitted as light that is visible to our eye, the rest is wasted as heat. If you hold the palm of your hand near a light bulb you will feel this radiated heat. So nine times as much energy is wasted as is turned into light.

Another way to produce light is to excite the electrons around atoms causing them to radiate light when the electrons change positions. This type of light is easy to spot. For example, sodium atoms give out yellow light (as in yellow street lights) and neon atoms give out red light (as in neon signs). Mercury atoms give out ultraviolet light which excites the atoms on the walls of fluorescent lights to emit a mixture of colours to form white light. This is a much more efficient process and so fluorescent lamps do not get hot.

Recent research has physicists using an inert gas which emits ultraviolet light with sufficient energy to have the phosphor emit two packets of visible light for each packet of uv light. They are searching for phosphors which emit three different colours to produce white light.

Over the last thirty years physicists have learned how to excite atoms in solids called semiconductors and can now manufacture light emitting diodes (indicator lights on electronic equipment) and lasers (in compact disc players) in which an even larger fraction of the electrical energy is converted into light. There will be many more developments in the near future as physicists learn more about such processes and how to make use of them.