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It is said that all digital devices these days emit blue light rather than UV; long term exposure can hurt our eyes. Why doesn't viewing the blue sky for a long time hurt us? Is there any big difference between these two blue lights?

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    $\begingroup$ How often do you spend any length of time looking directly at the daytime sky? $\endgroup$ Commented Sep 9, 2020 at 2:46
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    $\begingroup$ I agree with the previous comment. How do you know that long-time exposure to the blue skies above us will not hurt your eyes? Maybe people should be watching the blue skies above instead of their mobile and laptop screens below. -1 Oh, by the way, blue light is blue light. $\endgroup$ Commented Sep 9, 2020 at 3:03
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    $\begingroup$ I would imagine that any strain your eyes feel is due to focusing on small images/text for long periods of time....the same can happen from reading for too long. $\endgroup$
    – Triatticus
    Commented Sep 9, 2020 at 4:30
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    $\begingroup$ The health effects of blue light in particular are more related to possible disruption of circadian rhythms than eyestrain. $\endgroup$ Commented Sep 9, 2020 at 6:47
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    $\begingroup$ Why do you think the two kinds of blue are not different? $\endgroup$ Commented Sep 9, 2020 at 7:49

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Blue light inside earths atmosphere

The daytime sky receives its color due to Rayleigh scattering, as more blue light is scattered:

Scattering in percent of light

It is for the scattering, that the light that hits earth, thus, what you see, has less intense blue light. And thus is a little less harmful.

Experiments

To actually tell if a laptop monitors emissions are more harmful to your eyes than the blue sky, you will need to perform a few measurements and calculations:

You will need to measure the power (per area, at the distance you usually have from the screen) radiated by your screen, which depends on the settings, the currently displayed frame (its colors) and the type of screen you use. An example LCD display for instance has the following spectrum:

lcd intensities

You would need to also measure the power (per area) of the skys blue light on the ground to compare them.

Generally I'd say there is no reason to perform an experiment with your own eyes though.

Concluding

The damage your eyes experience depends on the time they are actually exposed to the specific light. I always find myself looking away from the blue skies after a few seconds as it usually feels too bright... and look at my PCs monitor for hours.

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    $\begingroup$ “It is for the scattering, that the light that hits earth, thus, what you see, has less intense blue light. And thus is a little less harmful.” I’m glad someone answered with a solid scientific mindset. While the word “scattering” in Rayleigh scattering would seem to imply the resulting blue light is weaker, however, your hypothesis is likely incorrect. The sun is very, very bright, so bright that the color camera sent to the moon during Apollo 12 was inadvertently destroyed when astronaut Alan Bean accidentally pointed it at the sun. There’s plenty of blue light left that makes it to earth. $\endgroup$ Commented Sep 9, 2020 at 9:37
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    $\begingroup$ I agree to your viewpoint. This means it is highly probable that sky's blue would hurt us if viewed for prolonged duration. $\endgroup$
    – Swami
    Commented Sep 9, 2020 at 13:56

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