…and so do cats and dogs. (Even hedgehogs.) And reindeer.
See in ultra-violet that is.
I’ve often wondered if humans can share this superhuman capability (a contradiction in terms as I realize). What’s the evidence? First the background…
The discovery of ultra-violet (UV) is an interesting story. In 1801 Johann Wilhelm Ritter read that William Herschel had discovered “heat rays”, a form of radiation below the red end of the spectrum (we now call these “heat rays” infra-red). Ritter looked for a form of radiation at the opposite end – beyond violet – and found one. Of course he could not see it but he proved its existence by shining white light through a prism and showing that silver chloride placed beyond the violet edge of the spectral colours still darkened, showing that there was a form of radiation – of light in fact – that we can’t see but silver chloride can.
It’s well established that birds and bees can see UV. Some birds have a specific visual pigment for UV and this extended vision plays an important part in their lives. Humans, and mammals generally, don’t have specific photo-receptors for UV. But it turns out that the three types of colour-sensitive cell we do have – responding to red, green and blue – can also respond to UV, with the blue-sensitive cells slightly better at it than the others.
So why don’t humans see UV and how it it that cats (for example) do? The reason is that in normal circumstances it is blocked by the lenses of our eyes, which filter it out and prevent it ever reaching the retina. Why this filtering takes place is not entirely clear.
One theory is that that our lenses are thicker simply because we are bigger animals. Cats are smaller so there is less lens to filter out UV.
A second is that our lenses are adapted to block UV in order to protect our retinas, which are more easily damaged than those of other mammals.
A third is that thicker lenses offer sharper vision which has importance for humans, whereas the vision of other animals may prioritize other aspects. For example cats have lower visual acuity but are much better than us at seeing in dim lighting.
Whatever the reason for the filtering, the effect is that most humans UV-blind.
But surprisingly not all. Some people can undoubtedly see UV. They may have thinner lenses, or no lenses at all (the term is aphakic), often as a result of surgery for cataracts, or artificial replacement lenses that allow UV through. They typically describe its colour as “whitish-violet”, which probably results from their red, green and blue receptors all being stimulated, which would give a sensation of white light, and the perception of whitish- violet comes from the blue receptors being somewhat more sensitive than the other two kinds.
And what about reindeer, why might it be useful for them to see UV? (Maybe Rudolph’s nose was the wrong colour!) Here’s a clip from University College London.