Why do our eyelids not block out all outside light when they are closed?
In: Biology
For the same reason you can see blood vessels under your skin at your wrists. Skin is translucent, it doesn’t block all light that hits it.
Hold a flashlight up to your finger and see how it seems to glow. That glow is light passing through all the tissue, much more than makes up your eyelid.
I think s/he means, why is the skin thin? Why has evolution made/kept that skin translucent?
The skin covering your eyes is translucent. The light you see is traveling through your skin.
Skin is so thin that your eyes can see the light that can still pass threw them. Like putting a flash light on your hand and you can see the density of the bones, ligaments, other junk.
I would guess that it’s for light detection to regulate our sleep-wake cycle. It’s important to know when the Sun is coming up to avoid predators who hunt at night since they’re more visible during the day, to go out and about before the Sun is fully up and it’s hot, etc.
If you want a more literal answer, it’s because our eyelids aren’t thick enough to block out all light. It’s also important to remember that humans didn’t arise when indoor lighting was a thing.
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