So one of our readers asked us this question the other day: Why do we see black spots after looking into a bright light?


The spots aren’t, strictly speaking, “black”, they are “no signal” areas. Sight is basically a chemical reaction. The rods and cones of your eyes produce dyes. (See “Rhodopsin” and/or “visual purple”) These dyes are how your eyes see.

Photons come in, intersect, and change these dyes. This is what produces the initial chemical stimulus that becomes the nerve impulse response to light. The photon(s) intersecting with the dyes “uses up” the dye. Your body is constantly making new dye and cleaning up the used-up dye.

When you look at a bright light it uses up a lot of dye. Then those rods and cones don’t have enough to really generate a good signal.

Similarly, if you’ve been in darkness for a while, you’ve got a lot of dye built up and you can see really well in the very dim light. Further the very dim light uses up very little dye and so you continue to see well in the dimness.

This whole mechanism is why the room looks “darker” right after you turn off the lights, and then “your eyes adjust”. It’s also why the “blue spots” move with your eyeballs because it’s the individual sensors that are exhausted.

Your eyes also physically adjust to darkness or brightness by opening or closing the iris, but that’s a different level of responsiveness. After two or three seconds the dark room will be somewhat more visible because of the iris adjusting, but it can take minutes for the dye levels to get good, and after half an hour the room might be really quite visually available. etc

So your eyes work on eye-fuel, and the more light they process the more of that fuel is exhausted. As it gets exhausted the cells that are exhausted put out less signal for the same light and “dark patches” seem to appear in your vision.

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Last Update: September 29, 2016

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