Sight beyond sight.

Our colour vision stretches from the longer wavelengths we see as red to the shorter wavelengths we see as violet. But ultraviolet vision is fairly common among insects, fish, reptiles and birds, especially those with smaller eyes that filter out less UV light. Bees have excellent UV vision thanks to colour receptors optimised for detecting it, but at the cost of poorer vision at the red end of the spectrum. UV vision is used for different purposes by different species, from kestrels detecting the urine trails of prey to reindeer spotting polar bears. Reindeer were thought rare among mammals in having UV vision, but Ron Douglas of City University London reported this year that many mammals, including hedgehogs, cats, ferrets, seals, pigs and rabbits have lenses that let UV through.

A few people can see into the near-UV spectrum. What do they have that the rest of us don’t? Well, actually, it’s what they don’t have. The receptors in our retinas can detect UV but wavelengths shorter than 400 nanometres are normally filtered out by the eye’s lens. People who develop cataracts and have their lens replaced with a UV-transparent one sometimes start seeing UV as a bluish or purplish glow. The painter Monet may have started seeing UV after a cataract operation at age 82, influencing his famous series of water lily pictures.

From New Scientist