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This question arose when I thought that on Normal Days, at least I have observed bright sun while going outside, going to office, in park etc. for less than a second. It happens almost daily. Not deliberately but that's how things work I guess. Similar example is playing cricket. Many times sun comes in the way of ball in the air and again I have a glimpse of sun. To me, it is very common as I've seen in international cricket matches on TV.

On the other hand, in case of the days of Solar Eclipse, on one or two occasions, when the sun wasn't very bright due to clouds, I've deliberately observed the partial eclipse just for a glimpse, just to confirm it's actually cut. And I confirmed it. (How fast it was? It was not like I stood still and observed the sun for less than a second and then closed eyes. It was like this: I started moving my head and sight from earth surface and in a faster way moved my head towards sun and without any break, stopped at safe location on earth again where there was no sun). And it did literally no harm to me.

So my doubt is:

Why there's so much theories that you must not stare sun even for less than a second or even a glimpse by mistake? If you see news on eclipse day, you'll see full news about this. But on any other normal day, no such news. Is there something special, something more bright on solar eclipse day? Will it damage eyes?

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    $\begingroup$ Closely related: astronomy.stackexchange.com/q/22233/16685 Note this comment: "The sunset also doesn't trick your pupils into dilating". $\endgroup$
    – PM 2Ring
    Commented Jun 21, 2020 at 19:18
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    $\begingroup$ On normal days, people are significantly less likely to attempt to spend minutes staring into the sun, and the damage the sun does to your retina is painless. $\endgroup$
    – notovny
    Commented Jun 21, 2020 at 19:21
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    $\begingroup$ When the eclipse is near totality, the sky is dark, so your pupils dilate, letting in more light. But the light rays coming from that small sliver of Sun are just as bright as normal, and have the same proportion of ultraviolet as full sunlight. I'll put this into a proper answer. $\endgroup$
    – PM 2Ring
    Commented Jun 21, 2020 at 19:55
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    $\begingroup$ @Vikas Yes. The retina has no pain nerves, so the typical reflex response "That hurts, I should probably stop doing it" is not generally in effect for retinal damage caused by looking at the Sun. $\endgroup$
    – notovny
    Commented Jun 21, 2020 at 20:02
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    $\begingroup$ Variants of this question have been asked multiple times on Physics Stack Exchange. Here is a starting place: physics.stackexchange.com/questions/353019/… $\endgroup$
    – Mark Foskey
    Commented Jun 22, 2020 at 3:19

3 Answers 3

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Glancing at a partial solar eclipse is about as dangerous as glancing at the Sun on any other day. If you look at the Sun a few minutes after sunrise or a few minutes before sunset, when the Sun's altitude is low, the light is filtered through a lot of air, and most of the ultraviolet is scattered, so it's a lot less dangerous than looking at the Sun in the middle of the day. However, during an eclipse, people tend to look at the Sun for more than just a few minutes, and that's not healthy for the eyes.

The big danger of looking at a solar eclipse without proper protection occurs when it's a total (or nearly total) eclipse. When the eclipse is near totality, the sky is dark, so your pupils dilate, letting in more light. But the light rays coming from that small sliver of Sun are just as bright as normal, and have the same proportion of UV (ultraviolet) as full sunlight. Those UV rays can cause a lot of damage to the retina very quickly, and that damage can be permanent.

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    $\begingroup$ slightly related (but about cameras): Are there industry standards or specs for image sensor resistance to damage from intense light? $\endgroup$
    – uhoh
    Commented Jun 21, 2020 at 23:47
  • $\begingroup$ Is there reason to believe it's any more damaging than stepping out of a dimly-lit indoor room with no windows into the outdoors and immediately glancing at the sun? $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Commented Jun 23, 2020 at 16:46
  • $\begingroup$ @R..GitHubSTOPHELPINGICE It's probably pretty similar. Or looking directly at the Sun through a crack in the curtains of a dark room. But I Am Not An Eye Doctor. $\endgroup$
    – PM 2Ring
    Commented Jun 23, 2020 at 17:53
  • $\begingroup$ I'm afraid I feel I was more curious about knowing if we look at the sun the way I explained in question, will it damage eyes? $\endgroup$
    – Vikas
    Commented Jun 25, 2020 at 14:01
  • $\begingroup$ @Vikas There's nothing happening on a solar eclipse day that makes the Sun more dangerous than normal, apart from the thing I mentioned about having dilated pupils because of the darkness when the eclipse is very close to totality. $\endgroup$
    – PM 2Ring
    Commented Jun 25, 2020 at 14:12
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This webpage gives a lot of background information. Key points:

  1. It does seem to be the overall dimness of near-total eclipses that allows the pupil to widen enough to allow damaging levels of UV in.
  2. There are cases of eye damage from staring at the full sun, and also cases of staring at the noonday sun without apparent damage (not recommended). Perhaps levels of pupil dilation were different for different people.

The linked article points to UV light as the main culprit in eclipse-related eye injuries. Please don't use this as an excuse to try viewing a partial eclipse through just UV filters. Improvised, "this-ought-to-work" filters have led to multiple cases of eye damage. I don't think you could get volunteers for a study mapping out in detail which intensities at which frequencies cause human blindness. Best to assume they all do when you are planning to look hard right at it.

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    $\begingroup$ Thanks for providing the missing sources for PM 2Ring's answer! $\endgroup$
    – I'm with Monica
    Commented Jun 22, 2020 at 8:54
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    $\begingroup$ Hmm, this page seems to imply that using a pair of photographic UV filters as glasses would drastically reduce the danger while preserving the colors seen on the sky and the ground (which are completely removed by a solar filter). $\endgroup$
    – Ruslan
    Commented Jun 22, 2020 at 21:19
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Disclaimer

I have no medical knowledge and do NOT recommend following what I describe below.

Any children that read this should not copy it. I was stupid and it was pointless. Especially do NOT look at the sun with a telescope. It will burn the back of your eyes and cause blindness.

When I was about 11, I was told not to look at the sun but not told why. Being a rebellious child I saw this as a challenge. In fact I used to deliberately look at the sun to 'prove' how clever I was. It was painful and for a while afterwards I got a sort of ache that appeared to come from the back of my eyes. Of course I also got afterimages for up to a minute.

I am now in my 70's and suffering no ill effects. It seems that it did no lasting damage.

Repeat: I do not recommend this. It serves no purpose and I'm lucky that I did not not blind myself. I did it from youthful ignorance.

Do not do it. If you accidentally glance at the sun for a moment, don't worry.

If you look at the sun through a telescope you will almost certainly burn your retina and blind yourself.

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  • $\begingroup$ If you accidentally glance at the sun for a moment, don't worry. Does it mean it won't harm then? Because I don't see any harm so far after years. $\endgroup$
    – Vikas
    Commented Jun 25, 2020 at 14:03
  • $\begingroup$ I'm no expert either. From what I remember bright visible light can bleach the photoreceptor molecules, but they are replenished eventually. All light can make heat and if there is thermal damage it can not be repaired. UV can also do nasty things, including cause cancer. More at Can no amount bright light blind you? which refers to Feynman's confidence that looking at a nuclear test (through glass I believe, to absorb the UV) would not permanently blind him. He probably estimated the heating also. $\endgroup$
    – uhoh
    Commented Jul 27, 2020 at 14:08

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