Photo: Broderick (flickr)
We humans can see only one segment of the whole spectrum of electromagnetic radiation, the segment we’ve cleverly named visible light.
Like humans, most animals rely on visible light for seeing, and plants rely on it for photosynthesis. In addition to the light that’s visible to us, the sun also radiates ultraviolet and infrared light.
Snakes have infrared sensors, and bees can see some ultraviolet light, but they’re among the few exceptions to the rule. Why are we earth dwellers so attuned to visible light, instead of some other segment of the spectrum?
Why Don’t We See Ultraviolet Or Infrared?
The traditional answer to this question was that most of the sun’s radiation is visible light, so our eyes evolved to see those wavelengths. However, the sun also gives off a whole lot of infrared radiation. What would it be like if our eyes evolved to see infrared light?
If we could see infrared, the sky would appear dark, and grass and trees very bright. Our veins would shine darkly through our skin, and warm blooded creatures would be very easy to spot. That would be a great advantage not only for predators on the prowl, but also escaping prey.
No one knows for sure why animals and plants on earth evolved to perceive visible light, rather than infrared. One theory is that maybe there’s no biochemical reaction that would allow us to translate infrared light into a visual image we can perceive, as we do with visible light. Or, maybe evolution just hasn’t found it yet.