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My prescription glasses come with free anti-blue reflective coating. The idea is to minimize the exposure to blue light, which studies have shown can be damaging to eyes over time (especially if it comes from a strong source such as sunlight).

The problem is, it works both ways. The sunlight behind me or from my side hits the inside of my glasses, which gets reflected into my eyes as focused blue light.

My question is whether this feature is actually doing more harm than good, since the majority of the day I am not trying to actively stare at bright things, so the bright things will tend to come from the sides or back rather than directly in front of my eyes.

And since apparently no scientist was ever smart enough to consider this rather obvious setback, there are no scientific studies on it that I could find, so I would appreciate those who are knowledgeable in the mechanics of eyes, light, and glasses to try to estimate an answer using the principles of physics.

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    $\begingroup$ I'm voting to close this question as off-topic because you need to consult an ophthalmist, not a physicist. $\endgroup$ May 24, 2018 at 22:46
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    $\begingroup$ I disagree with this voting because it is extremely unlikely that an ophthalmist, optometrist or other eye-specialist will be any more knowledgeable about this than a physicist. I've scoured the internet for scientific research on how damaging the inside reflections are, but there are zero scientific studies on the subject, which means that those in the industry have either never considered it or simply chose to ignore it. $\endgroup$
    – pete
    May 24, 2018 at 22:52
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    $\begingroup$ An ophthalmist will be a full doctor with a specialty in all things eyes. That includes any required physics knowledge. It would be rather surprising if they did not understand what you need to know and, more to the point, what effect all this could have on your vision in the short, medium and long term, something we won't know on Physics SE. In short, ask the relevant medical specialist who will also be able to factor in your personal medical factors. $\endgroup$ May 24, 2018 at 23:09
  • $\begingroup$ Don't you already have an answer to this question on Quora? $\endgroup$ May 26, 2018 at 18:12

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The amount of blue light captured by your glasses and reflected into your eyes in the "backwards" direction will be quite small compared to the amount blocked in the forwards direction. I can sketch out for you the reasoning using geometry and basic optics, but unfortunately the margins of this text are too small to contain it. This is a physics joke which relies heavily on irony, so it may be safely ignored with no loss of generality.

However, note that human eye exposure to ordinary sunlight in the absence of sunglasses of any type has been going on for hundreds of thousands, if not millions, of years. Almost all of that exposure occurred outdoors. The idea that human eyes need to be protected from blue light exposure is an extraordinary claim, requiring equally compelling evidence.

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  • $\begingroup$ 1. The coating material seems to be the same on the front and back. Thus intuitively it would seem that if the backward reflections are insignificant, so too must be the forward directions; in other words, backwards reflections aren't harmful IFF forward reflections aren't helpful. 2. I believe there's some studies about people who spend a long time outdoors with higher risk of eye problems. Humans also didn't brush their teeth for millions of years but that doesn't mean brushing teeth is useless; we live longer now. $\endgroup$
    – pete
    May 25, 2018 at 21:16
  • $\begingroup$ agree on the teeth thing. regarding the eye protection: the amount of light entering the glasses from "behind" is probably less than 10% of the amount entering from "in front". this backwards light has to come from nearly directly behind you in order to strike the lens; as such, almost all of it will pass through the lens and exit the front and only a tiny amount will be reflected off the lens and reach your eye. so, the damage risk caused by light coming in the wrong way is (small) x (tiny) = negligible. $\endgroup$ May 25, 2018 at 23:26

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