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Science

The electromagnetic spectrum

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White light can be split up into a spectrum of many different colours and visible light is just part of a continuous spectrum of electromagnetic radiation. Different types of electromagnetic radiation have different hazards and uses.

What is a spectrum?

The visible spectrum

a rainbow appears to come out of the prism

Refraction from a prism

Using a prism, you can split up white light to form a spectrum. (A prism is a block of glass with a triangular cross-section.) The light waves are refracted as they enter and leave the prism. The shorter the wavelength of the light, the more it is refracted. As a result, red light is refracted the least and violet light is refracted the most, causing the coloured light to spread out to form a spectrum.

The electromagnetic spectrum

Visible light is just one type of electromagnetic radiation: there are various types of electromagnetic radiation with longer wavelengths of light than red light and with shorter wavelengths than violet light. All the different types of electromagnetic waves travel at the same speed through space.

The discovery of infrared

Sir William Herschel. Credit: John Russell

National Maritime Museum London UK

British astronomer William Herschel (1738-1822) was making observations of the sun when he put coloured filters over his telescope in order to make his observations safer. He noticed that different coloured filters heated up his telescope by different amounts. Using a prism to break up visible light he put a thermometer in the different colours. He found that the temperature rose as he moved the thermometer from violet to red. He then measured the temperature where there was no visible light, at the red end of the spectrum. The temperature was the highest: he had discovered infrared.

The discovery of ultraviolet

Following Herschel’s work, Johann Ritter (1776-1810) tried to find invisible rays at the violet end of the spectrum. As part of the experiment he used silver chloride, which turns black when exposed to light. This happened fastest when exposed to the invisible rays at the violet end of the spectrum.

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