Why do we see blotches after looking at lights?

13 March 2011

Question

Why is it that when you look into any type of light for a long period of time that blotches of black appear when you look away?

Answer

Sarah - Well this is very similar to the effect that you get when you're standing there at a party and someone takes a photo, and you get those spots in front of your eyes from the flashes and you just can't see anything.

It's because of something called photo-bleaching and it happens to the cells in your retina which is the light sensitive bit at the back of your eye. It's populated with cells called rods and cones, and these are full of light sensitive pigments.

Normally you get light as particles called photons coming into your eye; these stimulate the cells by changing this light sensitive pigment, which is called retinal, into a slightly different form and they stimulate the cells to send electrical impulses to your brain.

But, if you get a really bright light - like a flash or if you look at the sun for a long time or even just a really bright light bulb - this sends the cells into overdrive and they get really overexcited, and it takes them a little while to calm down. So then when you look at a normal level of light, like a wall, you get these kind of black spots where you've been looking at light where the cells have been over stimulated.

Chris - So, you have lost the photo pigment in that particular area temporarily because it's been broken down by the photons of light hitting it, and when it regenerates, it comes back then you see normal again, but while it's regenerating up the pigment because of the overexposure, you see a less intense spot, so you therefore see a dark spot. It's worth noting actually that the retina is less active when light shines on it than in the dark. It's in fact deactivated by light rather than activated by light. It's rather paradoxical, isn't it?

[Note added in proof - you should never look directly at the Sun. The lens in your eye focuses light onto the retina in the same way that a magnifying glass can focus the Sun's rays to a spot and cause things to burn. You can burn a hole in your retina leading to permanent sight loss...]

Comments

Hey does this have a name? Not the scientific reasoning I mean when you look at light and you see blotches, does that experience have a name?

Hello, I am a structural steel welder, and it's not uncommon for me to spend a full 8 hour day under my welding mask. I always try and use the darkest shade possible to protect my eyes from the arc flash, but even then when I lift my hood and blink a couple of times, I will still see a small dark spot right in the middle of my vision where the arc of the weld is the brightest through the lens...

It worries me because I feel like that can't be good for my eyes...but I don't really know of anyway to stop from happening.

When our light sensing cells get over stimulated like that, is it hurting them?

So I as a joke stared into the sun for like a minute tops but didn't really squint my eyes and afterwards I got the usual side effect where there is blotches of light in my vision but this has lasted a few days now instead of a few minutes and im getting a tad worried, should I be worried or will this go away in some time?

I would scurry over to an optometrist and get my eyes checked. Anything more than a few days could be a worrying sign.

You should seee this a doctor or maybe not to do that again

If I look at a light or something for a few seconds I see dots of green. Why is that?

I have noticed that lately if I have been looking at my phone screen or computer screen for a while and then I look away from it everything is kinda colorful and splotchy. But then after about 5 minutes or so everything is back to normal. Why is that?

Thats normal. Try moving your phone / pc screen away from your eyes and take 1 minute breaka every 20-30 minutes. Also, try to work in natural light.

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