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A shady past

John Stringer investigates the history and science of sunglasses

While it's possible some children will wear sunglasses to protect their eyes, most will want to wear them because they think they're trendy and cool. Probably few of them will realise that sunglasses go back thousands of years.

Ancient Egyptians are supposed to have used pairs of ears severed from a respected member of society as sunglasses. Apparently, they beat them flat with a mallet, dried them, stretched them, and put them on a wooden frame to create history's first sunglasses.

True spectacles developed in parallel with sunglasses. The reading stone, invented about 3,000 years ago, was a glass sphere that was laid on top of the material that was to be read to magnify the letters. Spectacles were invented in Italy around 1284 by Salvino D'Armate but it's not just Mafia types who give shades an evil reputation. The Salem Times of August 18, 1692, reported that Hazel Bones, one of the defendants, wore a pair of shades under examination during the Salem witch trials and described them as "a menicing (sic) black oval across her eyes that suggested a close relation with the devil".

It was Britain's James Ayscough who made the first true sunglasses, however. His spectacles in the year 1752 had double-hinged side pieces. His lenses were both tinted glass and clear. This was because he felt that white-coloured glass created a glaring light that was bad for the eyes. He advised the use of green or blue glasses instead. His tinted glasses were not made to shield the eyes from the sun, but to correct vision problems.

A parallel invention was discovered during the exploration of the Northwest passage in 1853. The British adventurer Robert John Le Mesurier McClure found Eskimos wearing solid wooden blocks across their eyes with tiny horizontal slits in them. These were protection against snow and ice "so piercing, heartless, and uninterrupted that it can permanently paint the insides of one's eyes the same pitiless shade".

But it was American Edwin H Land who in 1929 patented sunglasses that reduce glare. Polarising is a process that cuts the light oscillating in one direction. The waves that make up light oscillate in many different directions - up and down, side to side, and every angle in between. A polarising filter, like the lens of a pair of Polaroid spectacles, allows light through that oscillates in only one plane. So light that is reflected from horizontal surfaces such as roads can be filtered out. If you put a second pair of Polaroids in front of the first and rotate them to right angles, it stops light oscillating up and down too. Virtually all the light is cut out.

Polarising glasses are sometimes sold with a piece of test material to turn in front of the lens, making the view dark and proving the glasses are genuine. As we become more aware of the dangers of bright sunlight more children may bring sunglasses to school. You can tell them these familiar devices have a long history and together you can explore their efficiency in protecting their eyes.

Pupil activities:

* Compare sunglasses subjectively by looking through them at the same light source. Compare them objectively by using a light sensor.

* Identify which are Polaroid spectacles by pairing them and turning one pair through 90 degrees.

* In design and technology, make frames and lenses to suit different individuals.

* In history, identify the contributions of different scientists to understanding light.

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