Why can humans see colours when a large source of light is present, but when there’s very little light we can only see black and white?
In: Biology
So, you’ve got eyes, right? And at the back of those eyes, you’ve got cells called rods and cones, which are responsible for detecting light. And inside those rods and cones, you’ve got molecules called opsins, which are *how* those rods and cones detect light. Those opsins do an interesting thing – when light hits an opsin, the opsin absorbs it. This charges the opsin with a tiny bit of extra energy, and when that happens, the opsin panics and flips itself over to get rid of the excess charge (the reason flipping itself over gets rid of excess charge is a whole other thing). When it flips itself over, it initiates a chain reaction of various molecules within the cell that ultimately leads to the turning off of a signal that tells the cell not to do anything. When the cell stops being told not to do anything, it figures that must mean it *should* do something, so the thing it does is sends a message down the optic nerve to the brain saying “yo brain, I’m seeing some light over here!” Now, you may be asking why the cell needs to turn off a “don’t do anything” signal instead of turning on a “do something” signal. The answer to that is that evolution is kinda stupid, and turning off a don’t do anything signal works just as well as turning on a do something signal, it’s just the thing that happened to evolve.
Now, light has two relevant properties here: Intensity – how much light there is; and wavelength – what colour the light is (also, note that colour is not an inherent property of light, it’s just an illusion our brains create to better differentiate between different wavelengths, which is how colourblind people exist: Their illusion creation mechanism responds to certain wavelengths in a different way). And there are also various kinds of opsins, 4 of which are relevant here. You’ve got rhodopsin, which is found in rod cells, and you’ve got 3 different kinds of photopsins, found in cone cells (each cone contains one kind of photopsin, and the three kinds are termed S-cones, M-cones and L-cones, for short medium and long). Each of these opsins responds to light in different ways based on the intensity and wavelength. Photopsins respond to wavelengths of their related length (short, medium or long), and so when they absorb their appropriate light, the signal sent to the brain says “yo brain, I’m seeing some short light over here!”. Photopsins also flip very quickly when they absorb light, so they’re very responsive and accurate. However, photopsins are also not very sensitive. It takes a lot of light absorption before the cone cell has enough to turn off the don’t do anything signal, which means they don’t send signals in low light conditions.
This is where rods and their rhodopsin comes in. Rhodopsin find itself unable to give much of a shit about the wavelength of light it absorbs. It will happily absorb light from every wavelength from short to long, or thereabouts. Because it responds the same to every wavelength though, the signal rod cells send don’t give any information about what wavelength of light is being detected, so the brain can only interpret that signal as a generic “there is some light here” signal. Rods are also very sensitive. It doesn’t take much light to set them off, so even in low light conditions, there’s often enough light to at least trigger a few rod cells.
This is a trade-off evolution makes. It has not yet found a way to make photopsins that are highly sensitive to light, so it can’t create low light colour vision, and rhodopsin is certainly better than nothing.
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