Snow blindness, arc eye, welder's flash, bake eyes—these all describe the common effects of staring at an intensely bright light source. But what actually happens to your eyes when you try to hold a staring contest with our closest star? It's not pleasant, that's for sure.
In short, it's a sunburn inside your eyeball. Of the three types of light that the sun produces—visible, infrared, and ultraviolet—UV is by far the most damaging to structures within the eye, especially when reflected off sand, snow or water. The cells of the cornea—the transparent outer layer of the eye—will blister and crack when overexposed to UV light, much like a normal sunburn. Symptoms of this condition, known as photokeratitis, usually appear a few hours after the damage has occurred but are easily identified by excessive tearing, tissue inflammation, and the feeling that you've rubbed your eyes with fine grit sandpaper. Luckily, these symptoms are almost always temporary, dissipating within 36 hours, and can easily be prevented by wearing properly UV rated eye wear. The eye's lens can also be damaged from too much UV light, typically resulting in cataracts and invasive tissue growth known as pterygium.
Damage to the retina—the collection of light sensitive cells located at the back of the eye that transmit images into your brain—however is much more dangerous. Solar retinopathy, as the damage is known, may not be painful like photokeratitis but the results can be permanent. When the light-sensing cells of the retina are overstimulated, they release a flood of signalling chemicals that can damage surrounding tissue if present in sufficient concentrations. So, technically, retina damage is more like a chemical burn than a thermal one. This condition is typically reversible over time—from one month up to a year, depending on the amount of damage sustained. In some cases, healing progresses steadily over the course of 12 months, in others it heals rapidly in the first month then remains static for 18 months before improving again.
However, even though the pupil will naturally contract when exposed to bright light, the amount still entering the eye is concentrated on a tiny substructure of the retina responsible for the majority of our central detail vision, the macula. If this tissue is damaged, it could cause macular degeneration resulting in permanent blindness in the center of your field of vision.
The Sun, of course, isn't the only source of hazardous UV rays in the modern world. Lasers, especially those that shine in the shorter wavelength blue to ultraviolet range, can wreak the same sort of photochemical damage that the sun does—albeit on a much shorted time scale. The amount of light reaching the retina from a laser source can be up to 200,000 times as intense as the sun and don't trigger the body's natural defensive "squint and blink" reaction (only the visible spectrum causes that).
Laser-based UV rays cause thermal damage in two ways, neither of which is particularly pleasent. The energy from a laser beam can heat up the cells of the eye until protein denaturation—the same biochemical process that hardens boiled eggs and firms cooked meat—occurs, which results in cell death. They can also heat water molecules present in the eye until they spontaneously boil, often with explosive results. Not "explosive results" like your eyes will burst forth from the ocular cavities, just a small popping sensation just before the world goes dark. And now you know why it's against the law to shine lasers into cockpits. [Wikipedia 1, 2 - Sight and Hearing - Livescience - Eye Journal: Solar retinopathy after the 1999 solar eclipse in East Sussex - Pak J Ophthalmol 2007 - Image: rangizzz / Shutterstock]