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. Here's a look at what you shouldn't see for yourself.
The moment you begin looking at the sun, you start to develop a sunburn on your eyeball. Of the three types of light that the sun produces—visible, infrared, and ultraviolet—UV is 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. It's a lot like a normal sunburn. Symptoms of this condition, known as photokeratitis, usually appear a few hours after the damage has occurred. They're identified by excessive tearing, tissue inflammation, and the feeling that you've rubbed your eyes with fine grit sandpaper. Luckily, the effect is almost always temporary, dissipating within 36 hours, and can be prevented by wearing UV-rated eye wear.
Look steadily at the sun for a little longer—like the girl on amphetamines who stared at a solar eclipse for a full half hour—and you can cause damage to the retina. This collection of light-sensitive cells located at the back of the eye transmits images to your brain. 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 signaling chemicals. In sufficient concentrations, like during a long look at the sun, these can damage surrounding tissue. 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.
With enough damage to the retina, though, staring at the sun can leave you partially blind. Prolonged UV exposure can damage the macula, a tiny substructure of the retina responsible for the majority of your central detail vision. The pupil will naturally contract when exposed to bright light, but the amount of light still entering the eye is concentrated on the macula tissue. Damaging it can cause macular degeneration, eventually resulting in permanent blindness in the center of your field of vision. Basically, that black dot you see after a photo flash would just never go away.
Permanent, complete blindness can also occur from staring at the sun over the long term. The eye's lens can be damaged from too much UV light, typically resulting in cataracts and invasive tissue growth known as pterygium. As UV-induced cataracts advance, they can obscure a patient's vision. If left untreated, they can eventually cause blindness.
So, staring at the sun without proper eye protection is a clearly bad idea, but doing the same through a telescope or binoculars is just plain dumb. As a magnifying glasses fries an ant, these devices concentrate the suns rays into destructive points, causing immediate photochemical and thermal damage. The UV can literally cook your eyes out of your head, destroying rod-and-cone structures and potentially resulting in permanent blindness. The only safe way to look at the sun through binoculars or a telescope is to install a solar filter. According to Dr. B. Ralph Chou, and associate professor at the University of Waterloo School of Optometry:
The Sun can only be viewed directly when filters specially designed to protect the eyes are used. Most of these filters have a thin layer of chromium alloy or aluminum deposited on their surfaces that attenuates both visible and near-infrared radiation. A safe solar filter should transmit less than 0.003% (density ~4.5) of visible light and no more than 0.5% (density ~2.3) of the near-infrared radiation from 780–1400 nm...The longer infrared between 1400 and 2500 nm does not get past the tears and front of the eyeball, so is not a problem.
If you can't afford a solar filter, you can always build a small pinhole projector or other indirect viewing method. You can also hold the binoculars a few feet in front of a white card and orient them towards the Sun. This will project a magnified view of the Sun onto the paper, allowing you to safely study its shadows during an eclipse.
So even if your mother wasn't exactly correct to tell you you'd go instantly, completely, and permanently blind from staring at the noontime sun, her warning was offering some useful advice just the same.
[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]