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Sometimes looking at a screen for too long hurts. This happens to me when I use my computer or smartphone for too many hours, and my eyes become tired and weak.

Lately I noticed that if, instead of looking directly at my computer screen, I look at its reflection through the covering glass of my turned-off iphone screen, not only it feels easier, the image actually seem to be sharper. I can actually notice the pixels.

What could be the reason?

Something that might be related: I also noticed that when I use my iphone or home computer, it is easier for me to look at it from aside ( > 45 degrees) instead of front. The screen suddenly seem more like a normal object and less like a light source.

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  • $\begingroup$ Perhaps you are farsighted? Typically, reflected images appear blurrier because they are further away. $\endgroup$ – Kyle Kanos Jan 9 '15 at 18:58
  • $\begingroup$ How old are you? If you are over 40 then you probably have presbyopia, even if you are not otherwise farsighted. $\endgroup$ – akrasia Jan 9 '15 at 19:15
  • $\begingroup$ @KyleKanos I am actually shortsighted, but I use contact lenses. $\endgroup$ – ipodppod Jan 9 '15 at 19:52
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    $\begingroup$ @akrasia I am 26. Either way, I don't think it's a matter of focus. It's more like the reflection is somehow filtered, making it more plasant to look at. $\endgroup$ – ipodppod Jan 9 '15 at 19:53
  • $\begingroup$ I just tried this, and the reflected image of my screen did look a little bit sharper, but that could have been somewhat of a placebo effect. It was hard for me to tell. I wonder if it might be that the reflected image is dimmer, and so bits of dust or smudges on your screen may not show up in the reflected image as well? $\endgroup$ – JotThisDown Jan 9 '15 at 20:20
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This is just a rewriting of the comments.

The significant phenomenon seems to be polarization. Dielectric media, such as the iPhone's glass, tend to remove the polarization in the plane of the incident and reflected ray, so that reflected light is partially polarized (or totally, if the angle of incidence is Brewster's angle).

Moreover the light produced by the LCD pixels is, ideally, polarized in a particular plane. So the polarization produced by reflection should remove a component of this light, more so than other sources of illumination, such as diffused light from the environment. Then it is reasonable that some details are enhanced, while others are suppressed, in a fashion that is not linear in brightness.

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I'm not familiar with iphones, so this is just a general answer: When looking at the sun reflected in a shiny surface other than a true mirror, the image seems sharper, probably because it is no longer too bright for your eyes. As a kid I used to look at hot burning lightbulbs via a white dinner plate and was amazed that one could easily read the text on the bulbs this way. It's almost like a party trick, telling people exactly how many watts the bulb is and even naming the brand without looking up!

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  • $\begingroup$ Cool trick! Still, I don't think the issue here is brightness because the screen and its reflection both seem to be equally bright. $\endgroup$ – ipodppod Jan 9 '15 at 21:38
  • $\begingroup$ Hmmm, then maybe it resembles enhancing the contrast of a television screen by placing darkish glass in front of it. The small reduction in brightness may not be noticable, especially if it is subjectively 'compensated' by the increased contrast. $\endgroup$ – Zaaikort Jan 9 '15 at 22:04

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