There's a reason that certain wavelengths are known as "visible" light, because those are the wavelengths that humans and most animals even remotely like us can see. But one mammal can see in ultraviolet light...and it's all because of snow.
On the light spectrum, ultraviolet fits in right between visible light and X-rays. It's invisible to the human eye, but we deal with its effects all the time - because it's in sunlight, it's a key part of what causes sunburn and other potentially damaging things to our bodies. Ultraviolet light is also what causes snow blindness, in which the eyes effectively get sunburned by the ultraviolet light reflected off of fresh snow. We'll be coming back to that in just a moment.
We know that a lot of our most distant relatives - birds, reptiles, insects, and so on - can see ultraviolet light, or at least wavelengths very close to it. But it was generally thought that mammals don't have eyes built to process those wavelengths. That's why recent research at University College London has turned up something very unexpected: reindeer rely on ultraviolet lights to see in the snowy expanses of the Arctic.
Lead researcher Glen Jeffery explains:
"We discovered that reindeer can not only see ultraviolet light but they can also make sense of the image to find food and stay safe. Humans and almost all other mammals could never do this as our lenses just don't let UV through into the eye. In conditions where there is a lot of UV – when surrounded by snow, for example – it can be damaging to our eyes. In the process of blocking UV light from reaching the retina, our cornea and lens absorb its damaging energy and can be temporarily burned. The front of the eye becomes cloudy and so we call this snow blindness. Although this is normally reversible and plays a vital role to protect our sensitive retinas from potential damage, it is very painful."
Since reindeer have to deal with Arctic snow for most of the year - which can reflect up to 90% of ultraviolet light - it makes sense that they would have developed the ability to see into the ultraviolet wavelengths. But it isn't simply a question of avoiding snow blindness. As Jeffery explains, their ultraviolet vision actually allows reindeer to see things that all other animals would miss:
"When we used cameras that could pick up UV, we noticed that there are some very important things that absorb UV light and therefore appear black, contrasting strongly with the snow. This includes urine - a sign of predators or competitors; lichens - a major food source in winter; and fur, making predators such as wolves very easy to see despite being camouflaged to other animals that can't see UV."
And, because all scientists are really mad scientists at heart, Jeffery closed with a little speculation on what everyone here is really thinking about - what's it going to take for us humans to start seeing into ultraviolet:
"The question remains as to why the reindeer's eyes don't seem to be damaged by UV. Perhaps it's not as bad for eyes as we first thought? Or maybe they have a unique way of protecting themselves, which we could learn from and perhaps develop new strategies to prevent or treat the damage the UV can cause to humans."
I'm calling it now. A superhero named Reindeer Man with the powers of ultraviolet vision will be tromping through the wastes of northern Scandinavia by 2015. Actually, that's sort of the premise of Hanna, now that I think about it.