Why aren't 100% UV blocked sunglasses safe to view an eclipse with?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

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.