system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: In bright light it hurts to open both my eyes fully, but if I close one eye, I can open the other eye much wider without pain. Why?
gpt
You're not actually feeling the intensity of the light with your eyeballs. The brain triggers the pain/wincing reaction in response to an overall level of white light being processed by the cranial nerve. The amount of light being brought in through one eye doesn't trigger that response, but the amount from both eyes does.

It's still not a good idea to stare at bright lights with one eye, though. The intensity of the EM radiation will damage the cone cells in your eye, regardless of whether or not it makes you wince.