Monday, February 19, 2007

Why Telescopes Use Mirrors Rather Than Lenses

In the 17th century, the English physicist Sir Isaac Newton recognised that there were problems with the traditional refracting telescope, which used glass lenses to focus light from a star

The lenses produced a fringe of false colours around the star. This happened because when a beam of light coming through glass is bent, its waves, being of different lengths, are bent at different angles. Blue light, for example, which has short waves, is bent (or refracted) more sharply than red light, which has longer waves.

So Newton designed a reflecting telescope which collected and focused light by means of two mirrors. (These mirrors were made from an ally of tin and copper, known as speculum metal.) The front of the mirror which collected the light was curved, rather like a shaving mirror. As a result, it could focus light just like a lens.

Large modern telescopes all use mirrors to collect light, although they have grown in size from Newton's 1in (25mm) mirror to a Soviet monster 236in (6m) in diameter.

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