Did your parents ever tell you not to stare directly at the sun? They were right. But there are other dangerous lights, too. If you stare at anything that emits a certain wavelength of blue light, you could quietly and irreversibly burn out your eyes.
Have you heard of blue light hazard? If you work outside, or with welding equipment it should sound familiar. People who work with arc-welders, who work in the sun, and who work under certain kinds of light have safety procedures telling them how long they can stare, and what eye protection they can use. If they haven't, they should, because blue light hazard is terrifying.
The Visual Cycle
In the retina of your eye sits an interesting little team, hand in hand. One member of the team is called an opsin. The other is called 11-cis retinal - or more generally called a chromophore. The two are connected, and passive, until a photon hits them. At that point, the 11-cis retinal goes through a process called isomerization, which is a fancy way of saying, "same atoms, different shape." The 11-cis retinal shifts around until it is in a new forme, called all-trans retinal. The opsin starts the process of phototransduction, meaning it changes the energy from the incoming photon to an electric signal that can travel along the nerves and get to the brain.
Meanwhile, back in the eyeball, the isomerization process has to be reversed for the eye to be able to properly receive the next photon. Groups of enzymes and proteins get the all-trans retinal back to 11-cis retinal, at which point an opsin can bind with it again, and it's ready for the next photon. The visual cycle is why you are occasionally blinded for a bit after an oncoming car flashes its high beams at you. You need a little while to re-set and get back to the proper form of retinal so you can see again.
Blue Light of Death
Something a little different happens when light of a particular wavelength hits the eye. Certain wavelengths between 380 and 500 nanometers in length - the blue and violet range - hit the eye and at first everything goes as normal. The team of opsin and 11-cis retinal goes through its change, activates the electrical signals that let us see the color blue, and then the retinal goes on to the next stage in the visual cycle.
But when the opsin starts up the process of phototransduction, certain intermediate products are made. When the intermediate products are exposed to more blue light, they can also bind with the opsin, and form a new little team that's ready to accept more photons. This process, the jerry-rigging of a functional photon receptor from intermediate products, is called photoreversal. It's a great deal faster than the visual cycle, and so it allows the eyes to absorb a lot more blue light than any other kind of light.
Photons Hitting the Retina
The eye is a delicate part of the body, and photons, small as they are, are units of energy. Too much energy will wreak havoc on the structures in the eye. Scientists have seen that too many photons can ruin an enzyme that helps with cell respiration - the process by which the cell makes energy and clears out its waste products. Too much exposure to light can also cause cells to produce singlet oxygen, an atom that tends to react with everything around it including proteins and DNA. Just to top things off, too many photons also crank up the production of hydrogen peroxide in cells. Basically, the repeated, rapid exposure to blue light causes cells to heat up and chemically destroy themselves.
Although blue light damage hasn't been studied in humans - for obvious reasons - it has been studied in rats and primates. Exposure to a great deal of blue light causes lesions in the retina of both animals. They were also, after enough exposure, permanently rendered color-blind in the blue-green range.
It's tough to prove the effects of blue light on humans, for many reasons. For one thing, we actually need a little blue light in order to keep our eyes healthy. For another, blue light damage - barring anyone voluntarily staring at the sun for hours - slowly adds up over a lifetime. It's hard to run controlled experiments over the course of a lifetime, especially when an errant glance or two can muddy the results. For now, people are warned to be careful when staring at the sun, or light sources that give off strong blue light. Listen to your mothers!