Is the problem with seeing an eclipse that you have a sudden change between penumbra and regular sun? If that's the case, why can I look to the sun with my sunglasses on but I can't do it in an eclipse because it would be harmful? Even if my sunglasses have an UV filter it would be bad for my eyes to see the eclipse with them?

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    $\begingroup$ "why can I look to the sun with my sunglasses" - I won't recommended it, I doubt the filters on your sunglasses have been designed to the quality where they can be used to safely look at the sun. $\endgroup$ – Quantum spaghettification Aug 20 '17 at 18:50
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    $\begingroup$ Do not look directly at the sun, ever, even with sunglasses. $\endgroup$ – DanielSank Aug 20 '17 at 19:00
  • $\begingroup$ Related: physics.stackexchange.com/q/11087/2451 $\endgroup$ – Qmechanic Aug 21 '17 at 14:07
  • $\begingroup$ Duplicate of physics.stackexchange.com/q/352914 ? $\endgroup$ – SAH Aug 23 '17 at 1:05

Both your blink reflex and the pupillary constriction reflect depend on visible light, but the danger to your eyes comes from the total energy delivered in the visible and the near infra-red.

The energy density of sunlight is higher in the near infra-red than in the visible.

The un-eclipsed sun is bright enough in the visible to forcefully trigger both reflexes and give you partial protection (unless you act the part of the pig-headed fool and stare into the face of the sun despite your body's warnings to the contrary).

A partially eclipsed sun can carry more than enough total energy to cook your retina while the visible light isn't bright enough to fully engage the reflexes, making it easier to do something stupid.

Don't do something stupid.

The same issue results in the significantly different danger classification of visible and infrared lasers of modest power.

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    $\begingroup$ Important part: if the total amount of light is less the pupil will not constrict as much. For those parts of the sun that are not obstructed the power per unit area on the retina will consequently be bigger... $\endgroup$ – Floris Aug 21 '17 at 2:06

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