Science Questions

Why do we see blotches after looking at lights?

Sun, 13th Mar 2011

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George Pope asked:

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?


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.  Itís 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.  Itís populated with cells called rods and cones, and these are full of light sensitive pigments.  Normally, in a normal situation, 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 itís been broken down by the photons of light hitting it, and when it regenerates, it comes back then you see normal again, but while itís regenerating up the pigment because of the overexposure, you see a less intense spot, so you therefore see a dark spot.  Itís worth noting actually that the retina is less active when light shines on it than in the dark.  Itís in fact deactivated by light rather than activated by light.  Itís rather paradoxical, isnít it?


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Make a comment RD, Thu, 10th Mar 2011

George Pope asked the Naked Scientists: 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? What do you think? George Pope , Tue, 15th Mar 2011

What does it mean when the blotches last unusually long and/or are triggered by relatively dim flashes (ie:the shine off of a clean car or a traffic light). Ashia, Wed, 5th Nov 2014

Drugs (prescribed or "street") or disease which delays/restricts pupils constricting could be responsible ... RD, Wed, 5th Nov 2014

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