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I am not planning on staring into the sun during an eclipse or any other time.

I have been reading about how no variety of regular sunglasses are safe enough to view the eclipse with. I'm not talking about being able to see things clearly, but just actual eye safety.

From what I understand it is the ultraviolet light that causes damage to the retina, but maybe it is more complicated.

How do my eyes get hurt if I am looking at the sun through so called "100% UV protection" and what makes the eclipse glasses sold in stores different?

edit: To clarify this is not about how the rays from the sun are dangerous, but about why "100% UV blocking" sunglasses fail. Do other dangerous rays get through? Is the "100%" marketing? Essentially, in what way are the best consumer sunglasses inadequate for looking at an eclipse.

Answers about pupil dilation and what makes an eclipse more dangerous for naked-eye viewers are not what I'm after.

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    $\begingroup$ The UV400 standard is to block 99% of UV. I suspect 100% blocking is a marketing figure, not actual. $\endgroup$
    – BowlOfRed
    Commented Aug 21, 2017 at 5:28
  • $\begingroup$ btw NASA allows you to view the totally eclipsed sun wihtout glasses. eclipse2017.nasa.gov/safety $\endgroup$
    – akostadinov
    Commented Aug 21, 2017 at 7:59
  • $\begingroup$ Possible duplicate of Are dangerous rays emitted during Solar Eclipse? $\endgroup$
    – Dmitry Grigoryev
    Commented Aug 21, 2017 at 9:38
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    $\begingroup$ In the 1999 total eclipse in Central Europe I've seen it directly, after some precautions. I didn't focus directly in the Sun, rather I scanned a line on the sky, what intersected the Sun, from horizon to horizon. The goal was to disperse the destruction on my retina, from a single point to a line. Furthermore, I did it quickly, it lasted only roughly a half second. Well, I admit it was still too dangerous, but fortunately I didn't experience any damage to my vision. And I've seen the eclipse with my own eyes. Now I wouldn't do it already. $\endgroup$
    – peterh
    Commented Aug 21, 2017 at 10:50
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    $\begingroup$ @peterh, people who don't take safety seriously when playing with low power lasers can do a lot of damage to their eyes before they begin to realize they have a problem. If you burn one small spot or one thin line on your retina, you won't "see" it. Your brain edits that spot out of your view of the world. It's only when there are a lot of missing spots and lines that you start to realize that words on the computer screen are somehow, not quite as easy to read as they used to be, etc. $\endgroup$
    – Solomon Slow
    Commented Aug 21, 2017 at 13:39

3 Answers 3

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You are correct that almost always it is the UV content of sunlight and not its power that is the main hazard in staring at the Sun.

The lighting during a total eclipse is one of those situations outside the "almost always". Eclipses did not weigh heavily on our evolution, so we are ill kitted to deal with them.

Moreover, UV sunglasses are not designed to attenuate direct sunlight, only reflected sunlight.

Normally, the eye's pupil is shrunken to about a millimeter diameter in bright sunlight. This means that it admits about a milliwatt of sunlight, which, for healthy retinas, is nowhere near enough to do thermal damage (see my answer here for further discussion).

During an eclipse, the pupil dilates to about $7\,\mathrm{mm}$ diameter to adapt for the low light levels of the eclipse's twilight. Thus its aperture is fifty times bigger than it normally is in sunlight. This means it admits a great deal more UV than normal (and the corona, at $100\,000\,\mathrm K$, radiates a great deal of this). You're getting about $50$ times the dose you would normally get even looking directly at the Sun.

Furthermore, suddenly the diamond ring phase begins, and high levels of sunlight reach the retina before the pupil can shrink again. The latter happens only very slowly. So even thermal damage is a risk here.

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    $\begingroup$ So UV of sunglasses does not filter it in fact? And how are the special sunglasses better? update: NASA allows you to view totally eclipsed sun without glasses: eclipse2017.nasa.gov/safety $\endgroup$
    – akostadinov
    Commented Aug 21, 2017 at 7:56
  • $\begingroup$ During a total solar eclipse, the only thing visible is the corona. It is visible because of Thomsen scattering of photons off of free electrons. Though the corona radiates the most strongly in UV, the light you see at Earth during a total eclipse has been scattered by 90 degrees. I am not sure how much UV is scattered by 90 degrees (from radial) in the corona but the total flux from the corona is tiny compared to the solar surface. $\endgroup$
    – honeste_vivere
    Commented Aug 21, 2017 at 14:39
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    $\begingroup$ I understand the issues with the pupil, but I'm not seeing where regular "100% uv" sunglasses fail. Do they not actually block 100% UV? Do they let in near infared, and can that damage you? $\endgroup$
    – Jeff
    Commented Aug 21, 2017 at 15:58
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    $\begingroup$ @Jeff I used eclipse glasses to look at the sun. They filtered so much light that I could not see anything through them except for the sun, and it was perfectly comfortable to do so. That is how they are different than sun glasses: they block far more of the light. $\endgroup$
    – jpaugh
    Commented Aug 21, 2017 at 20:04
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    $\begingroup$ @Matt Chambers unless not 99% of the light, but 99% of the spectrum are blocked - far fetched, but unless told otherwise, it would make a valid "99%" marketing claim :) $\endgroup$
    – rackandboneman
    Commented Aug 21, 2017 at 22:49
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The damage to your eyes comes from the total energy from the visible and near - infrared region even when you wear a 100% UV blocked sunglasses.

When you look at the sun in normal days, the visible light from the sun itself is enough for your eyes to trigger pupillary constriction and blink reflex in order to give you at least partial protection.

But when you look at an eclipsed sun, the light and energy from the infrared region will be more than the light from visible region. So no pupil constriction and blink reflex to save you. And the energy from IR rays will burn your eyes.

So it is unsafe to watch an eclipsed sun even with sunglasses, whether they have UV protection or not.

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    $\begingroup$ And how are the special sunglasses better? $\endgroup$
    – akostadinov
    Commented Aug 21, 2017 at 7:54
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    $\begingroup$ @akostadinov, the special sunglasses attenuate all wavelengths. Normal sunglasses have little or no attenuation of near infrared wavelengths. That's usually not a problem because even a really dark pair of sunglasses does not attenuate enough of the visible wavelengths to allow you to comfortably stare at the sun. $\endgroup$
    – Solomon Slow
    Commented Aug 21, 2017 at 13:30
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    $\begingroup$ Can you provide a source for the claim that "actually the damage to your eyes comes from the total energy from the visible and near-IR region"?Others have posted that much retinal damage comes from UV, but I've read multiple sources that say that the adult cornea and lens absorbs almost all UV light (ncbi.nlm.nih.gov/pmc/articles/PMC3144654, photobiology.info/Rozanowska.html). Thus retinal photothermal damage must come from visible and IR light. $\endgroup$
    – Matt Chambers
    Commented Aug 21, 2017 at 16:06
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    $\begingroup$ ncbi.nlm.nih.gov/pmc/articles/PMC3116568 - Conclusion of this project says - The protein of eye lens is very sensitive to IR radiation which is hazardous and may lead to cataract. sciencing.com/infrared-light-effect-eyes-6142267.html - Infrared B can cause damage to your cornea, iris, and lens; Infrared C causes damage to your cornea. Are these sources enough? $\endgroup$
    – Kawin M
    Commented Aug 21, 2017 at 17:48
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    $\begingroup$ @MattChambers Moreover, I didnt said UV is not harmful to eyes. But this question says Why aren't 100% UV blocked sunglasses safe to view an eclipse with? When you wear a 100% UV blocked sunglass, the damage to the eyes comes from the IR region. Thats why special eclipse glasses are used which claims to block 100% UV, 100% IR, 99.99% intensive visible rays. $\endgroup$
    – Kawin M
    Commented Aug 21, 2017 at 17:52
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The reason that sunglasses (even so-called 100% UV protection ones) aren't well-suited to protect your eyes from staring directly into the sun (at any time), is because sunglasses do not block the light, it polarizes it, i.e sunglasses allow all (at least most) of the vertically polarized light to go into your eyes.

The polarization filter is basically tiny slits in the sunglasses that allow light with the same orientation to pass through as depicted on the image below. Polarization

The reason sunglasses are made this way is because they are designed to filter the light reflected off of the ground and into your eyes.
Because this light is being reflected off of a horizontal plane, the reflected light is always horizontally polarized and thereby easy to remove.
If sunglasses just dampened all light, your pupils would just dilate to compensate for the apparent darkness, giving you no apparent benefit from the sunglasses with respect to your ability to see clearer.

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    $\begingroup$ What has polarization has to do with UV filtering? $\endgroup$
    – akostadinov
    Commented Aug 21, 2017 at 7:53
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    $\begingroup$ Only polarized sunglasses work this way. Most sunglasses are not polarized. UV filters on sunglasses do not rely on polarization as far as I can tell; they rely on sharp cut filters: google.com/patents/US8210678 $\endgroup$
    – Matt Chambers
    Commented Aug 21, 2017 at 15:48
  • $\begingroup$ A polarization filter does not actually have "tiny slits." The illustration above is only a schematic representation. Polarized sun glasses are made with Polaroid film---a plastic film in which millions of tiny polarizing crystals are embedded. $\endgroup$
    – Solomon Slow
    Commented Aug 22, 2017 at 16:52
  • $\begingroup$ @jameslarge There are wire grid polarizers, though they have wires arranged perpendicular to the direction of the light that is allowed through. $\endgroup$
    – Random832
    Commented Aug 22, 2017 at 17:57
  • $\begingroup$ @Random832, Yeah, I read about those after I posted my comment. While they do have rows of slits, their operation is based on how photons interact with valence-band electrons in the metal, and not about how the waves "fit" through the openings. Anyway, it seems kind of high-tech (at least, the ones that work at optical wavelengths are high-tech) for sun glasses. $\endgroup$
    – Solomon Slow
    Commented Aug 22, 2017 at 18:39

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