According to the electromagnetic spectrum, humans can only see visible light. We need special devices in order to see things like infra-red and ultraviolet waves. How come humans are born and are only able to see the visible light waves?
In: Physics
Mostly because that’s the part that’s most useful and it’s the part that’s most abundant. But there are some other problems sensing light outside of our visible spectrum.
Our sun peaks its output in the yellow-green part of the spectrum and as you can imagine that’s the part we’re most sensitive to. There’s just a lot of it, so that’s the most useful part to be looking for. A lot of UV gets absorbed and blocked by the atmosphere, and of course there’s almost no UV at night.
As well, a lot of wavelengths like radio and higher UV tend to just go straight through matter. That might *sound* useful but if you’re trying to look at *this thing* and the light is going through it, you’re not going to see it. Or you’re going to see multiple overlapping shadows and you’d have to figure out which of them on top.
Regardless, visible light is also kind of unique in how it interacts with light. Photons give electrons energy, right? In the visible spectrum, the energy is enough to move electrons up into higher “orbits” around their atomic nuclei, but only temporarily, which is good. That shift in electron orbit will change the shape of the molecule that atom is in, which is how we detect light. The photon energizes an electron inside of a protein, and that protein changes its shape. That interacts with other proteins, which causes the nerve to fire.
Above visible, the energy can knock the electron off entirely. That’s bad, because it breaks your detector protein. That means every time you detect a UV photon, you’ll probably have to rebuild your detector. And also, if that photon hits something *other* than your detector, it’ll break that too. So it can break your other detectors, other parts of cell in your eyes, and generally mess up everything. It’s much safer to block all of it out with proteins deliberately built to safely absorb and block that dangerous UV.
Below visible light, there’s not quite enough energy to move the electrons up. Instead, they kind of wiggle around like a duck bobbing on a wave. That’s harder to detect, and when you do, it takes a lot of detectors working together. Between that and the larger wavelengths, the vision you get is a lot less precise. That’s why things that detect infrared, like pit vipers, use it to sense heat in a more general “it’s over there” kind of way instead of a precise, “I can see this thing right here” way.
Humans evolved to see light in the frequency range most commonly emitted by the sun. We call it “visible” because we see it, not the other way around. Being able to see infrared and ultraviolet would require us to evolve new photoreceptor chemicals, ones almost no other mammal has. It would take a very, very long time.
I think this is more a question on categorization rather than biology. We categorized “visible light” because it was visible to us humans.
The cones in human eyes only allow us to see specific wavelengths of light.
Why do we not have cones to watch all other kinds of light? Because being born with those cones didn’t give enough of an evolutionary advantage (or rather, being born *without* those cones didn’t give enough of an evolutionary *dis*advantage) that there would be pressure towards having those cones.
And it’s not that we can only see visible light waves, it’s that we only call the light waves we can see visible. The universe didn’t decide that the wavelengths we can see should be visible. Plenty of animals can see outside the range that humans can.
Because the definition of “light” is the waves we can see.
just like “sound” are the waves we can hear
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