Even the dimmed sunlight from the solar eclipse can pose a danger to your eyes
Odds are that you’ve never directly viewed a solar eclipse, and you probably shouldn’t start any time soon for the sake of your eyeballs. While the eclipse does have interesting effects on our atmosphere, there’s nothing about the Moon blocking the Sun that magically transforms good sunlight into something dangerous. The sunlight is actually always dangerous, but most of the time it’s bright enough to remind us not to try and gawk at it. Even what seems like a small amount of light can be a health hazard to your eyes, so it’s very important to protect your peepers from the sun when things go dark on August 21st.
Our bodies are bathed in sunlight whenever we’re outside, and it’s obviously not such an immediate problem. Most skin can withstand short exposure to ultraviolet light (UV) without too much wear and tear, and our eyes handle the indirect UV light pretty well (although wearing sunglasses is certainly a good idea.) The reason this all compounds when viewing an eclipse is the that you’re looking right at the sun, and that light can be focused through the lens of your eye. Like a magnifying glass focusing sunlight to start a fire, your lenses focus light on the back of your at the retina. The intensity of directly-focused sunlight can quickly damage your cells by creating reactive molecules called free radicals, which then go on to kill the cell.
Safer ways to stare at the Sun
In most cases of this kind of damage, the damage is somewhat limited. The retina will basically be left with gaps where cells have been killed, and you will have a new set of blind spots in your eye to contend with. Sometimes people recover from this damage, but sometimes they’re left legally blind, as they can only see with the peripheral vision that wasn’t torched by the sun.
This isn’t to say that the only way to enjoy an eclipse is to avoid it. While your sunglasses are in no way up to the task of protecting your eyes when viewing an eclipse, solar-viewing glasses are designed to only allow a safe amount of light, meaning around 0.00032 percent of normal sun exposure. Alternatively, you can view the eclipse in the same way you usually take in sunlight— indirectly. A simple pinhole camera will let you safely watch a projected image of the Sun as it gets blocked out, all without staring right into the sky. If you’re looking for a closer look, don’t use your favorite telescope or binoculars unless you have specific filters for that as well, since that’s basically focusing sunlight at your retina even more effectively than your own eye’s lens can do.
My third grader asked: Isn’t the sunlight blocked enough to be less of a problem?
It takes very little sunlight to harm your eyes, especially when it’s being focused into your eyeball. However, once the Moon completely blocks the Sun during totality, it’s recommended that you take off your protective eyewear, as things will otherwise be too dark to see. With luck, you’ll get a peak at the Sun’s atmosphere around the outer rim of the Moon, and this light won’t be coming directly at you to cause harm. As soon as the Moon starts to move out of the way though, get your glasses back on since any direct sunlight can be a problem.
Source: If the Sun Is 93 Million Miles Away, Why Can't We Look Directly at It? by Rachael Rettner, Live Science