When you look directly at a bright light, why does it “stain” (for lack of a better term) your vision when you look away?


When you look directly at a bright light, why does it “stain” (for lack of a better term) your vision when you look away?

In: Biology

Your eye adjusts to normalize light coming in. So when you look at a bright light those receptors will temporarily become less sensitive to that specific type of light. That’s why it usually shows the opposite color to what you were looking at.

Photoreceptors, the things in your retina that “see” the light that enters your eyes and send it to the brain for processing, can only handle so much light before they have to “recharge”. Typically, they’re able to recharge fast enough to keep up with the light that enters them, which is why you’re not always dealing with light spots. However, when a ton of photons enter all at once, such as when you glance at the sun or peer into a lamp, your photoreceptors get overwhelmed and spend all their light-receiving molecules at once, so they take some time to recharge. The vision center of your brain isn’t able to process nothing, so it fills the gap in your vision in with the “white noise” from the photoreceptors around the empty spot, which is why you see that white-ish blob or spots.

Here’s a good post about it from a medical blog https://www.google.com/amp/s/www.huntervision.com/blog/bright-spots%3fhs_amp=true

Edit: to the people complaining that a 5 year old wouldn’t understand this, please read the sidebar

Bleached photo pigment.

When you light enters your eye. It is absorbed by a chemical (photopigment) and changes shape. When enough pigments change shape, it sends an electrical signal to your brain. This signal tells you there is light. These pigments are constantly changing shape (upon the detection of light) and being reverted back to their original shape so they can again absorb light again. When you stare at a bright light source, you’re bleaching or changing more photopigment than that part of your eye can regenerate. This results in an afterimage. Most people take approx 10 mins to regenerate that photopigment. This is also the reason why it takes around 10 mins to adapt to a dark room.

Disclaimer: not a scientist, just some dude with a vague interest in biochemistry.

At the heart of the cells in your eyes that receive light from the world and produce a signal to the nerves is a chemical (similar to betacarotene) that’s continuously reacting back and forth between two forms. This is a dynamic equilibrium, where the relative concentration of both of these chemicals changes when exposed to light.

When you stare into a light for a long time, you heavily skew the equilibrium towards charged molecules, and it takes some time for the signal to revert back to its baseline.