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, 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 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?

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