Why do bright lights blind us?

Our eyes work when light breaks down a chemical in them. Increases in light levels break down too much chemical and blinds us for a moment. A drop in light levels leaves us short of chemical so we can’t see until our eyes make more of it.

We’ve all been there; in that temporary blindness following flash photography. That echoing image of a bright light that stays on your eye for a while, even after the light has gone. But why does it happen?

Everybody knows that it takes our eyes time to adjust to different light levels. We stumble around blind for a few moments after turning the lights off until our eyes adjust. We take a few seconds squinting after we turn the lights on until the light is no longer painful. It is this same phenomenon which is to blame for our “flash blindness”.

Diagnosing the problem

Let’s do an experiment. Go into a dark room and let your eyes adjust to the dimness. Now put a hand tightly over one eye, so you can’t see out of it, and turn the light on. Let your open eye adjust to the light. Once it’s done turn the light off again. Now uncover your other eye…

Weird isn’t it? One eye can see fine, but you feel like you’re blind in the other one. This means that this isn’t your brain playing tricks. This happens within the eyes themselves and each eye can act independently of each other. 

How the eye works

What actually happens is that the effect isn’t even confined to the entire eye. A TV makes up an image out of lots of little dots of colour called pixels. People refer to TVs as being 1080p, 720p or 4K (4000p). This refers to the number of pixels that run across the length of the screen. Our eyes work in a similar way, but instead of tiny lights we have cells on the retina in the back of the eye. Each cell works by itself. Our brain builds up a picture of the world by putting all those little specks of light together.

As the cells work alone, they adjust to light without listening to their neighbours. So when a camera flash goes off in your face it is the cells that see the flash that need the most recovery. As they recover, they do not work as well as the other cells and this is why we see that echo of the light source in our eye.

Extra curricula info:

For those of you who would like a bit more information, here you go.

The cells in our eyes send their information to the brain when a molecule (known as a photopigment) is broken down by the light hitting it. The eye constantly replaces its stocks of this molecule so it is not a problem until the levels of light change. If the light levels go up, suddenly there is too much of the chromophore in your eye and it all gets burnt off at once by the new light (so we squint to try and slow the process, to make it less uncomfortable). Once enough has been burnt off we start to feel normal again.

Likewise, when light levels drop we suddenly don’t have enough chromophore in the eye for low light to register. Now all that we can do is to wait for the eye to produce enough chromophore for the low light levels to start breaking it down again. Until this happens we are effectively blind.

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Matilda’s Lab is written to explain complex science to children. Effort is made to keep the content accurate but sometimes it is necessary to oversimplify things. However, if anything is factually wrong, please do get in touch so that a correction may be made.