Stories about 'how things got invented' generally start off with everyday experiences. Donald Glaser, the inventor of the bubble chamber – a device used in particle physics – is supposed to have been staring into a pint of beer when inspiration struck. Rather than finding it at the bottom of a beer glass, other inventors (if you'll pardon the pun) have illumination thrust upon them. Percy Shaw, a 1930s road-repairer from Yorkshire, was probably one of these.

Eureka!

The story - or at least one of them - begins as he was driving home late one night. The eyes of a cat caught in his headlights alerted him to the fact that he was about to veer off the road (and, as it happens, down a precipice). He swerved to avoid the cat and in doing so probably saved both its life and his own. This anonymous moggie unwittingly became the preserver of countless other people, both in Britain and throughout the world. Shaw was a clever man who used this experience to create a simple road safety device that went on to make him a millionaire.

How Does It Work?

Catseyes are road studs that contain 'retroreflectors': optical devices that reflect light rays back along a path parallel to that of the incident ray. There are essentially two ways of making a retroreflector. The first, and easiest, is to make a transparent sphere and to 'silver' the back of it. This makes the sphere act like a lens with a mirror at the back of the focal plane, which so happens to be the back surface of the lens. Light enters the sphere, gets refracted, bounces off the back surface and then leaves the sphere on a path parallel to the incoming ray of light¹. This is what happens in the eye of a living cat: the mirror is the tapetum, a thin layer of reflective cells at the back of the retina that amplify its response by trapping light.

The other kind of retroreflector is the 'corner reflector'. It comprises three reflecting surfaces positioned at right-angles to one another. A light ray bouncing off three of these mirrors in sequence will eventually get sent in exactly the opposite direction². This kind of reflector can be made from mirrors or, if you're working with radio waves, sheets of metal³. Another method is to embed lots of tiny cubic facets on to one surface of a transparent block, leaving the other face smooth. In this type of reflector, light enters the smooth face, and then bounces around the back surface as a result of a phenomenon called 'total internal reflection'. It then emerges from the front surface again. Corner reflectors were left on the moon by the Apollo missions so that astronomers could measure the distance to it by laser rangefinding, yielding an answer accurate to within a few centimetres.

Shaw's Method

When Percy Shaw first invented the catseye reflector, the only transparent material he had to work with was glass. There was no prospect of machining hundreds of cubic facets on a mass-produced road-stud, so he opted for the spherical retroreflector. He also knew that any mass-produced device, embedded in the road surface, had to be almost totally maintenance-free and therefore self-cleaning. He therefore took further inspiration from its living analogue: by mimicking the actions of both eyelids and tear glands. He mounted four of the glass spheres, two pairs facing in opposite directions, in a raised latex moulding which in turn was set into a cast iron 'shoe'. This was then asphalted into the centre of the road. The master stroke was in the way these devices cleaned themselves. The shoe was designed to hold rainwater, and the moulding had a lip positioned just below the two glass spheres: the main part of the housing could be deformed to move past it. When a car drove over the stud, it depressed the housing into the shoe, causing the lenses to wipe on the lip. It also compressed the rainwater held in the shoe below the moulding and caused it to squirt upwards, cleaning the spheres as it did so.

Production

Shaw invented his prototypes in 1933. He then set up a company, called Reflecting Roadstuds, and in 1935 opened a factory to manufacture these. A year later, he placed the first 50 at an accident blackspot near Bradford. Unlike their modern counterparts, they were only placed at the edge of the road.

In 1937, the UK's Ministry of Transport ran a competition to identify the best road reflector. After two years, there was only one runner left in the race (Shaw's invention); the others had either broken or were no longer reflecting. Soon, virtually every major road in Britain had them fitted, and the device was also exported throughout the world. The most common catseyes on British roads are found in the centre and are white. On motorways, red catseyes are placed on the hard shoulder and amber ones on the central reservation. Green catseyes denote a junction and police drivers will notice the blue variety leading off their own reserved sliproads on to the motorway.

The Legacy

Despite amassing a huge fortune for himself, Shaw still lived in the same house in West Yorkshire until his death in 1976. He spent his money wisely, keeping a cellar full of White Shield beer for his frequent parties - but declining to have curtains or carpets fitted. His contribution to road safety was recognised in 1965 when he was awarded an OBE⁴.

In the latter years of the 20th Century, better plastics enabled corner reflectors to be produced easily. Modern catseyes are rigid plastic studs. These are not self-cleaning and need to be replaced much more frequently than Shaw's version. Also, 'intelligent' solar-powered road studs, equipped with light-emitting diodes, are now being fitted to some roads in the UK and other countries. Whether these will work as well as Shaw's 'brilliant' invention remains to be seen, particularly when driving at night.

Related Websites

• Visit The Nobel Prize website to read a biography of Donald Glaser.

• The Physics Classroom has an in-depth explanation of 'total internal reflection'.

• The UK Patent Office is an essential site for aspiring inventors.

• In 2002, BBC Radio 4's Today held a poll of favourite and least favourite inventions.