## The wavelength and temperature of electromagnetic radiation

All hot objects emit electromagnetic radiation but the precise frequency and wavelength of that radiation depend on the temperature of the object.

Let's think about a hot lump of coal. The coal will emit a wide range of wavelengths – some visible, some ultra violet and some infrared. At high temperatures there will be a large amount of energy and much of this will be emitted in the visible part of the spectrum. As the coal cools down there will be less total energy emitted per second, less visible light and more infrared. When the temperature has fallen still further the coal will only emit infrared – on a dark night you would not be able to see it.

The graph in Figure 1 shows how the energy emitted per second by a hot object varies with wavelength and frequency. There are two lines on the graph – one shows an object at high temperature and the other the same object after it has cooled down.

(Remember that long wavelength means low frequency. Long wavelength is at the right hand side of the graph and high frequency at the left hand side).

Notice how the area under the lines changes from when the object is hot to when it is cool, and also how the position of the wavelength where most energy is emitted per second moves towards the long wavelength side.

Stars behave in some ways just like the lump of coal. In the constellation of Orion you can see two bright examples of hot and cold stars.' The red giant Betelgeuse is a cool star while the blue giant Rigel, a really hot star, is bluish white. The surface temperature of Rigel is about 10 000 oC while that of Betelgeuse is 'only' about 3400 oC.

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