While explaining to my nephew about the physics of light, I told him we cannot see infrared color, and he kicked back with a very simple question: why can't we see it? I could not tell him. Is the human eye unable to sense such light or can it indeed but the brain can't understand the signal? any other reason?
A nephew-friendly, physics-based explanation:
Our brains and nerves work based on electrical impulses, which are little bursts of electrical current. Electricity is what happens when you remove the electrons from one atom or molecule and move them to another one nearby. In some materials, like metals or heavily ionized liquids like blood, it's easy to move electrons around and make electrical current flow. In other materials, like plastic or rubber or bone, it's harder to make the electrons move.
It takes energy to make an electron move away from an atom. In conductors, it takes only a little energy; in insulators, it takes a lot of energy. How much energy it takes to liberate an electron is called the "work function" or the "ionization energy," depending on exactly what you're doing, and is measured in volts. (Well, technically it's electron-volts, but that compound word makes people fall instantly asleep.) If you push the same number of electrons --- the same current --- out of a nine-volt battery, you do about six times the amount of work of a 1.5-volt AA battery.
If you hit an atom with some energy but it's not enough to knock the electron completely free, you can sometimes make the electrons around the atom vibrate. But the atom can't vibrate any old way: only certain frequencies are allowed. If you try to give an atom energy in some amount that's not allowed, the atom's electrons just ignore you. It's kind of like finding a vending machine that says "quarters only": if you have a pocketful of dimes and dollar coins, then too bad for you.
We happen to live in a world where ionization energies for stuff are typically three or five or ten volts, and electronic excitation energies are typically one or two or three volts.
Light is the way that electric charges exchange energy with each other. Light comes in lumps, called "photons," which each carry a certain amount of energy. It turns out that the energy in each lump is directly related to its color: violet light has more energy per lump than blue, blue more than green, green more than yellow, yellow more than red, and red more than infrared. When visible light hits the pigment proteins in the retina, it makes the electrons vibrate; that sets in motion the machinery to send an electrical impulse to your brain. When ultraviolet light hits those pigment molecules it ionizes them, which makes the molecules fall apart and sets in motion a different mechanism ("cleanup on aisle four"). And when infrared light hits those pigment molecules, it doesn't have enough energy to make the electronic vibrations go, so you get zero information about the infrared light: you're at the vending machine, but only with dimes. Visible light photons have energies from about 1.8 volts (red) to about 3 volts (violet).
The whole story is more complicated than this because the different ways a molecule can vibrate depend very sensitively on its shape, but that's the basic idea. This is also why ultraviolet light is more dangerous than visible light: in addition to breaking up pigment molecules, ultraviolet photons have enough energy to break up DNA molecules.
Infrared light can make an entire molecule vibrate, which is what we call heat. (It's easier to make a whole molecule vibrate because molecules are big and floppy, while the electrons are held near their atoms on a short, stiff leash.) The pit snakes have a delicate membrane which seems to detect radiant heat by causing warmed air to flow through a pore; you can see right away that this thermo-mechanical sense is completely different from the electro-optical method that we (and the eyed snakes) use to see visible light.
Infrared radiation is absorbed by water, both atmospheric water vapor and liquid water.
Below is a graph of water transmission at various wavelengths. Notice that some rather large bands are completely missing. This light can't reach your eyes because the air absorbs it.
Also, our eyeballs are filled with water. This water also absorbs infrared radiation before it hits our retinas. Below is a graph of the how much light is absorbed by liquid water by wavelength.
This explanation also explains why we can't see UV light. Here's a graph of all wavelengths we could see versus how much can be transmitted through air.
Essentially, our eyes evolved to see visible light because that's the only light there is to see.
Hot objects, such as humans and warm-blooded animals, emit IR-radiation. Few creatures have IR-vision, such as snakes. Significantly, snakes are cold-blooded. IR-vision would be advantageous in a struggle to survive for most creatures; with IR-vision, you could distinguish (hot) living creatures with camouflage from their backgrounds and see (hot) living creatures at night.
There could be many obstacles to evolving IR-vision - you must consider whether IR-sensitive eyes could evolve in gradual, incremental advantageous steps. However, given that snakes have IR-vision, whatever obstacles exist must be surmountable.
The big problem, I think, is that humans are warm-blooded. That means the head behind the eyes and the eyes themselves (I assume the eye is also at body-temperature), emit IR-radiation. This background noise might make it impossible to have a useful IR-sensitive eye. It would be like trying to read a book while someone shone a bright torch into your eyes.
We might have been able to get around this by having eyes outside our bodies, like a strange antennae, by evolving a sort of goggle that transforms IR- to visible light at the front of our eye, or maybe be some kind of shielding behind the IR-sensitive cells. I think these things are probably sufficiently "irreducibly complex" to make them very difficult to reach by evolution.
This might only be a problem for IR-radiation of particular wavelengths similar to human body temperature. Perhaps we could see other IR wavelenghts. However, since most warm-blooded animals are similar temperature to humans, $\sim37^\circ$, there would be no advantage to seeing such wavelengths, so it is no surprise that we did not evolve this adaptation.
The human eye is indeed not able to sense such light(*). The retina, which basically covers more than half of the internal wall of the eye has 3 different types of cones (which are photoreceptors, i.e. cells that will transmit an electric signal to the optic nerve if they are excited by light they can sense) that are only sensitive to visible light. The reason of why they are sensitive to only visible light lies in that the cones have a size and proteins that make them well suited to aborb visible light only and not UV, infrared and other EM radiation of other wavelengths.
Visible light corresponds to the portion of the electromagnetic spectrum ranging from roughly 400 nm (violet) to 700 nm (red). Infrared on the other hand corresponds to wavelengths greather than 700 nm and up to 1 mm.
(*) sidenote: The real story is a little bit more complicated, apparently the human eye can detect an infrared laser with a 1064 nm wavelength emission, but it appears either red or green due to some complicated process ocurring on the retina.
Look at the Plank Spectrum for a blackbody with a temperature of our Sun (roughly 6000 Kelvin). Here is a link to one.
From my understanding, the reason we cannot see outside the visible range is that our eyes evolved to match the spectrum of our sun. So for example with solar systems with much hotter stars, where the peak of the black body spectrum is in the ultraviolet range, it is conceivable that life evolving around such a star would likely see in the ultraviolet range.
For the same reason we cannot wag our tail
Ask him: "Can you wave your hand about?"
Answer: "Yes, like so!"
Ask him back: "Can you wag your tail?"
Ask: "Why can you wave your hand about but not wag your tail?"
Answer: "Because I have a hand to wave but I have no tail to wag."
It is the same thing with infrared light and visible light: we can see visible light because we have things (cones and rods) needed to see it. But we cannot see infrared light because we were not given anything to see it with.
"But why were we not given that?"
"For the same reason we were not given a tail: it just did not happen".
If he presses the issue you will have to start explaining evolution. Mutations and changes happen at random, and if a change is useful to us in a way that makes us more fit, then the change sticks around and will eventually become more pronounced. So somehow this mutation either never happened, or it did and it just was not very useful and faded away.
...which is the case with our tail that we do have. Our ancestors had a tail, and us humans have it too while in the womb. But the tail in a fully grown human has gone away over time because it was not needed.
I stumbled upon this thread while looking for a source in which I once read that opsins evolved to detect light from UV (in insects) all the way to the red part of the spectra is due to the nature of light. It is not only, that it coincides with the pure abundance of the photons but also with the energies they carry. The short wavelengths can be detrimental and for an organism, that reaches sexual maturity around 13 years and has to raise its young for at least a few years the effect would be that we would go blind and would have difficulties in continuing our line. Insect don't have those problems as they live a shorter life. So this sets the useful lower limit. As wavelength increases the energy of photons falls. In the range around 700 nm it becomes so low, that the opsins would have to be so close to the tipping point of conversion that the thermic vibration would be a problem.
The UV photons have too much energy, while IR has too little for a well enough built opsin molecule to efficiently detect it.