Slit pupils allow colourful nights

If you have ever wondered why your cat's eyes have slit pupils rather than round ones, scientists may now have the answer.

Researchers from Sweden have found that our feline companions have slit pupils because the design enables them to see sharply focused colour images when prowling around at dusk.

Ronald Kroger and Tim Malmstrom from Lund University, Sweden, report that it is not only cats that have stumbled across the same evolutionary solution to a common optical problem - many other animals have too.

The trouble is that single-focus lenses such as those in humans suffer from chromatic aberration. This means that different wavelengths of light are focused at different distances from the lens and, as a result, some colours are blurred.

In the latest issue of the Journal of Experimental Biology , the researchers reveal that many animals solve this problem by using multifocal lenses.

These are composed of different refractive zones in concentric rings, with each zone tuned to a different wavelength.

Almost all animals with multifocal lenses have slit pupils, which help them to make the most of their unique lens, according to the paper. This is because, even when contracted, a slit pupil lets an animal use the full diameter of the lens, spanning all the concentric refractive zones, allowing for all colours to be sharply focused.

When round pupils, such as those in human eyes, constrict, they cover the outer refractive rings of the lens, preventing the focusing of certain colours.

Professor Kroger and an international team of researchers received the Rank Prize in opto-electronics in 2004 for discovering multifocal lenses in an African cichlid fish.

To find out how common multifocal lenses are among animals, the researchers went on to examine zoo animals and found that many species have such lenses.

"Multifocal lenses aren't a freak solution for a few species," Professor Kröger stressed. He hopes this natural solution to an optical problem may inspire improved man-made optical designs.

"Multifocal optics are unheard of in optical engineering," he said.