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Note: I had asked this on Physics, but it is off-topic there due to being about safety.

On the BBC's guide to eclipse-watching, Dr Lucie Green says:

Watching an eclipse with normal sunglasses provides virtually no protection. In fact, they trick your eyes to let in more light, so they can cause even more damage.

This doesn't seem right, because proper sunglasses should block UV to a safer level. Surely wearing proper sunglasses at any time is safer than not? Whilst I can understand staring at the sun with sunglasses on still allows too much UV in, why couldn't someone just glance at the eclipse from time to time?

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closed as off-topic by fileunderwater, March Ho, Amory, Corvus, Chris May 17 '15 at 7:21

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Personal medical questions and health advice are off-topic on Biology. We cannot safely answer questions for your specific situation and you should always consult a doctor for medical advice." – fileunderwater, March Ho, Amory, Corvus, Chris
If this question can be reworded to fit the rules in the help center, please edit the question.

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Your eyes can dilate behind sunglasses, especially when the main source of light (the sun) is blocked, and ambient light decreases. Regular sunglasses offer some protection against certain UV wavelengths, but not enough to protect your eyes when looking directly at the sun (or, during an eclipse, the corona). Most sunglasses are mainly designed to, first and foremost, look good. After that, there is some protection against UVA (400-315 nm) and UVB (280-315 nm) light. Some sunglasses also offer "UV400" protection, which is short-wavelength blue light longer than 400 nm. Polarized sunglasses do not protect against UV at all, only reflected light (such as shining off the water or from an automobile's body or glass) and glare. Different sunglasses tints (or colors) also do not protect against UV, but they can affect color perception.

Surely wearing proper sunglasses at any time is safer than not?

That is true, it is safer, the question is to what degree? Sunglasses offer some protection from indirect UV and visible light that has been scattered by the atmosphere and reflected off of things around us. However, the number and energy of photons (the "particles" of light) reaching your eyes when looking directly at the sun is dramatically greater, and so much more dangerous. Eyes can be damaged quite quickly by high-energy light of many wavelengths, including UV and visible, with the magnitude of the damage correlating to the energy of the light. Damage is also cumulative, accruing over a lifetime. Even small peeks directly at the sun will contribute to your overall lifetime risk of eye damage, and may cause permanent, irreversible harm very quickly.

There are a number of much safer alternatives for watching solar eclipses, including pinhole projectors, welding goggles rated higher than 14, various solar filters for cameras, telescopes, and binoculars, and fully exposed and developed black-and-white film (good luck finding that nowadays), among others.

Above all, make sure you're safe, and if you have any doubts consult with an expert, such as a science/natural history museum or local university's astronomy department.

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  • $\begingroup$ So, if this answer biology.stackexchange.com/questions/34286/… is correct, then we should avoid looking in the general direction of the sun at all times? $\endgroup$ – binaryfunt May 16 '15 at 19:38
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    $\begingroup$ @BrianFunt yes, that is correct. $\endgroup$ – MattDMo May 16 '15 at 19:41

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