OK, so everyone knows that looking directly at the sun is bad.
But here in Southern California yesterday, the sky was smoky enough that the sun appeared as a red disk in the sky; not at all bright. I could look right at it, and could actually see the sunspots on it. It was pretty cool.
I told my friend about this and said he should check it out. He told me that it could damage your eyes, no matter how bright it actually appeared.
I was always under the impression that the brightness of the sun was what was actually damaging; Is there something else (e.g. invisible radiation) that could have damaged my eyes from looking at it yesterday? Are we just asking to be blinded whenever we look at a sunset?
It’s a bit of a judgement call. While you were probably safe, it’s not in the public interest to encourage people to stare at the sun under any circumstances. Sure as god makes little green apples, some chucklehead would do it when there’s too little filtering, and end up blind.
The Bad Astronomer (Phil Plait) has a chapter about looking at the sun in his book Bad Astronomy. Basically, looking at the sun is not as dangerous as most people think, but it’s still not a great idea. It’s normally so bright that most people will look away from pain before doing permanent damage. Also, the brightness will cause your pupils to constrict, which tends to minimize the damage.
However, during a solar eclipse, the amount of visible light is low enough that you can stare at it without physical pain, and your pupils remain relatively dilated, making the damage that much worse. But the amount of ultraviolet during an eclipse is enough to do permanent damage. Cheap sunglasses may cause a similar problem. Cecil Adams on Do old or cheap sunglasses increase the risk of eye damage from ultraviolet rays?
I don’t know offhand if smoke reduces UV radiation enough to make it safe. I wouldn’t count on it.
The whole reason things looked red is from the Rayliegh scattering, which is frequency dependent. The UV should be scattered even more than the visible. So it’s no different, mechanism-wise, than looking at a sunset.
I used to stare directly at the midday sun on regular noncloudy days. Not for long, but definitely long enough to resolve the image (to see the sun head-on as a disc, that is). Probably didn’t do my eyes a hell of a lot of good. “Children – do not try this at home”, etc. The point is that your ability to look at the sun because it was not too bright to look at doesn’t mean much, insofar as I’ve always been able to do it.
I have encountered exactly the same thing - a very hazy sunset that produced a very pale red disk, so I could see the sunspots. I thought it was cool, and posted a thread on here about it, and loads of people said “DON’T LOOK AT THE SUN! EVER!”. Well, being a scientist I reasoned exactly the same as swansont - the UV light is the harmful stuff, and the blue (and therefore UV) end of the spectrum is all filtered out, hence the red sunset.
So I say don’t worry about it! And enjoy the naked-eye sunspots, they are cool.
This is true of a partial solar eclipse, or the phases of a total eclipse before or after totality, when you can see the actual disk of the sun. But during totality, it’s perfectly safe, because all you’re seeing is the corona; the harmful rays are blocked by the moon.
By the way, one sure way to damage your eye is to look at the sun through a telescope or telephoto lens, without the proper filter. You’ll lose a noticible amount of sight within hours.
According to an article by a doctor of optometry at http://www.mreclipse.com/Special/filters.html, long-term UV exposure can cause cataracts, but it’s visible light and near-IR that can destroy the retina.
The key thing to note from the eclipse article is that even material that is very dark to VISIBLE light, like multiple layers of sunglass lenses or photo films, can be essentially TRANSPARENT to infrared radiation, which will painlessly fry your rods and cones. Solar filters are designed with either a metal coating or dyes that reflect or absorb infrared in addition to reducing visible light by a factor of more than 100,000. The dust in the atmosphere at sunset might reduce IR some, but I wouldn’t be willing to bet that it cuts it enough to be safe. Anybody who tries to see sunspots with his naked eyes is taking a huge risk.
To the extent that IR can hurt your eyes (or any other part of you), it won’t be painless. IR does damage by heating, and you can feel heat. Which leads to the over-obvious warning that if it hurts, don’t do it.
But if the Sun is visibly reddened (meaning that the UV is filtered out) and it doesn’t hurt to look at it (meaning that the visible and IR are at manageable levels) then there’s no harm in looking at the Sun.
That comment just got me wondering… what do you mean exactly by “fry”? Is it because photons physically smash into the rods and cones and damage them? Or is it because the stimulus is so intense that the rod’s and cone’s reaction causes the damage? I can’t see how it could be the latter, because if the rods and cones are reacting to IR, wouldn’t we perceive it?
Frying by heat. The cornea of the eye can focus IR radiation longer than 1 um (don’t remeber the actual range now). The mode of damage is the same as for visible light, the light gets focused and heats the retina. This can cause bleeding and other damage, leading to partial blindness. Most likely the partial blindness will be right at the center of vision where it causes the most trouble.
Side note: some types of damage to the retina can be reversed at the hospital if you seek help quickly enough.