What happens to 'light' after it enters our pupil?

Basically there are two sorts of processes which happen in physics, and the "particle picture" governs the difference between them.

Light can be absorbed by an atom as a whole, giving it a "kick," making it move faster. This is a "thermal" process; the light gets absorbed as intermolecular motion.

Light can also be absorbed by one of the electrons on the atom, giving that a "kick." Usually this knocks it to a higher-energy orbital around the nucleus, so that the atom is in an "excited state". But if the energy is high enough, it can also remove the electron completely, which we call "ionizing" the atom -- the atom becomes a charged ion. Either way, we physicists call these "chemical processes."

Which happens depends on the energy of the photon. Atoms can only be ionized at certain discrete levels, and if the photon doesn't have enough energy to make the atom "jump" from 1 to 2, then it cannot produce chemical changes, it can only heat the atoms up.

The formula for the energy of a piece of light is E = h f = h c / λ. Here h is a special number of quantum mechanics called Planck's constant, f is the frequency, c is the speed of light, and λ is the wavelength of that light. We usually speak of light in terms of wavelengths, and we usually measure energy for electrons in terms of "electron volts" or "eV", which is the energy gotten by an electron moving through one volt of electric potential. Atoms' excited states are typically in the ~1 eV range or so.

Now, there is this useful way physicists look at temperature, which is like this: temperature describes an average energy present in any "degree of freedoms' in which an object can move. This average energy is around k T, where k is Boltzmann's constant and T is the absolute temperature. For a room at 300 K = 27°C = 80°F, this energy is around 26 meV; in the human body it's only a little higher, at 26.8 meV.

In other words, if there are any chemical changes happening at less than 0.05 eV or so, then they would be happening *routinely* due to thermal variations. That is why there is this fundamental scale of ~1eV for important chemical changes; just a bit under this at ~0.1eV, you begin to transition to a state where thermal excitations dominate.

The microwaves in your microwave oven are very long -- 10 cm across or so! -- and this formula says that they have an energy of 10 μeV, so it's really in the millionths of volts. These energies are much much smaller than those required for chemical changes, so your microwave oven doesn't chemically change the food inside; it just heats it up with a very bright light. Radio waves are even longer than microwaves, so even lower in energy than this. Cell phones and WiFi also use microwave frequencies, so they can't give you cancer or any of that nonsense; the most they can do is warm you up -- they cannot cause any chemical changes which temperature fluctuations don't already cause.

On the other hand, typical light might be 500 nm, which is 2.5 eV. Now you see that chemical changes are happening! But of course they are; that is how our eyes work! Our eyes are biochemical systems -- they need to go into these "excited states" because that changes the configuration of some protein, which then triggers some long neuron to fire, transmitting a signal down the optic nerves into our brains

Ultraviolet light on the other hand might be 300 nm (which is UVB), which in the above formula is 4.1 eV. This stuff, we know to cause cancer. Specifically, skin cancer, if you get very large dosages from spending long long days out on the beach without sunblock on. Why does it cause cancer?

Because it has just enough energy to start damaging DNA.

DNA, as you know, is a double helix. This is very important because it means that when one strand breaks, the cell can repair it, as long as the other strand doesn't break anywhere nearby. Each strand "holds the other together." But with a lot of time, coincidences happen when both strands break. Usually, this just causes the cell to be unable to make some critical protein and therefore it dies. When lots of cells die, this becomes 'sunburn' -- you turn red because your body is putting a lot of bloodflow to your skin to heal it; you feel pain because your nerve cells detect that all of these cells around them have died, and you get very itchy because usually the dead skin cells are under half-living ones and your body wants to get this dead crap out of its skin.

But every once in a while, the change is far worse. Instead of just killing the cell, it causes the cell to replicate *out of control*. And this is what is happening to cancer sufferers.

I'm approximately at my character limit, so I will just say that while in physics energy is conserved, photons' numbers aren't conserved and instead we just think that the photon disappears when it is absorbed.

First of all Einstein' Theory of Relativity and so many others are not only wrong but false. If you base your explanations from what you have read but not understood, then one would be sympathetic. But to propound on some way out theory relying upon someone else's wrong assumptions and theories, you miss the mark altogether and the discussion becomes heated with religious philosophies and quotations form each others books. That is not how it is to be approached. So let us approach it from a different angle. Such a phenomenon cannot be explained but has to be experienced. What is real in the physical world is not the same in the non-physical (universal energy). What may be unreal in the physical material universe is totally different in the actual reality. the Universal energy. One has to study Tantra in its deepest sense of the word to understand the movement of energy within the material universe and the cosmic universe. Whatever happens at material death is that the body decomposes with active ingredients as insects and larvi aid the decomposition. So there is still motion and energy movement within that body. Once The Soul leaves the body, there is neither consciousness nor any recall of what happened. All theories about this that and the other are to be read with a pinch of salt... Scientists, medical professionals and metaphysicians give an account of neuro-muscular electrical impulses that subside and revert to their original state of non-being. Whatever else is there to discuss? Let us have some more views and get this thing off the ground in a sensible way for good knowledge to spread.... Thank you..

So, take a look at the eye diagram in the source box.

When the light hits your cornea, it bends into the pupil. Note that the cornea is convex, so the light bends inwards.

When it goes in the pupil, it goes through the lens, which makes it bend inwards again.

In order for you to see clearly, the light must join together and make a focal point on the retina, which is basically a wall on the back of your eye.

Extra notes:

The optic nerve is your blind spot. Your other eye makes up for it. If you put two dots on a wall, and back up a foot or two, close one eye. Then, with your open eye, look at the dot on the opposite side (look across your nose). The other dot should disappear. You've found your blind spot. ;D

Hope this helps!

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Check out, " Reception of Photons in the Eye " (Google).