Your brain is a very sophisticated computer. You’ll notice you actually can see red through your closed eyelids, if you look at the lights.
What happens is your brain filters out the one eye that’s seeing all red as unnecessary information and only utilizes the eye that can see a full picture.
Try closing one eye and holding your hand over the other, but far enough that you can see light leaking around the edges. You’ll notice the eye you closed is actually “visible” now as a reddish blob.
Our brains are really good at filtering out constant signals like the hum of machines or say the tick-tock of a clock. We only notice changes to our environment, and with a closed eye there is no change to notice. It’s kinda like how your nose is in your field of vision but you don’t notice it 99.9% of the time.
Another part of this is due to overlapping fields of view. Consider this: open just your left eye; then close it and open just your right eye; then alternate back and forth. While there’s a little difference, you’re seeing mostly the same stuff, right? Each eye, with the exception of what’s on the periphery, sees almost all of the same scene content as the other (i.e. they have largely overlapping fields of view). With one eye open, we still see something like 95% of what we see when both eyes are open. With both eyes closed, our field of view is 100% reduced; however, with only one eye closed, our field of view is only marginally reduced. Therefore it makes sense that we wouldn’t notice the “blackness” of one eye being closed as easily as we would when both eyes are closed.
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