Science Questions

Is the world much brighter than we see?

Sun, 17th Aug 2008

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Question

Brian K, North Yorkshire asked:

After having a cataract removal operation I went outside with my eye still fully dilated and I found myself nearly blinded by the strong light. Is the world really that bright and our eyes shut down and let in just enough light that we can manage it or is what we normally see the real reality?

Answer

Sarah - First of all, what we need to say is the world is constantly bathed in light from the sun, most of which we canít actually see anyway as itís in the ultraviolet or infrared ends of the spectrum. Weíll concentrate on visible light for now.

 

Your retina does have a range of brightnesses over which is works best so your eyes are always adapting to let in just the right amount of light - this is why when you look at someone in a dark room their pupils will be really big and dilated. When youíre outside on a sunny day they shrink to very tiny points.

 

In fact your eyes do something even more exciting than this. You may have noticed when you go inside from being outside in the bright daylight that it takes your eyes a while to adjust to the darker light levels. This isnít just the time it takes for your pupils to dilate. Itís actually something to do with whatís going on in your retina.

 

When you see light a chemical called rhodopsin which is in the cells in the back of your eye changes to a different shape which is how we see the light. As this gets converted into this different shape it gets used up and you canít convert it back very quickly. When youíre sitting outside in a bright light itís getting converted really quickly and itís really quickly running out so when you go into the dark thereís not enough of it left for you to be able to see really well in the darkness.

 

So the controlling factor is not the mechanical opening up of your pupil but the chemical change in the back of your eye.

 

Really the answer to Brianís question is that you are seeing slightly less light than the world produces but itís because your eyes and your brain have evolved that way to get the best out of them.

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Brknndy@aol.com asked the Naked Scientists: Question for the Naked Scientists: After having a cataract removal operation I went outside with my eye still fully dilated. And not surprisingly found myself nearly blinded by the strong light, the world appeared completely bleached out. My question therefore is: Is the world really that bright and our eyes shut down and let in just enough light that we can manage it? Or is what we see normally the real reality? I enjoy the show very much by the way. Hope you can help. Brian Kennedy North Yorkshire What do you think? Brknndy@aol.com , Sat, 26th Jul 2008

Hi Brian

yes, absolutely! The pupil works like the iris on your camera (hence the name is the same). By opening up the camera aperture very wide more light hits the film (or CCD), making the image brighter. This is useful under low-light conditions, but if you over-expose the picture, or accidentally expose the film to room light by opening the back of the camera, you get a white-out.

It's exactly the same with your eye; the retina contains photoreceptors, which are light-sensitive nerve cells analogous to the silver grains on a piece of film, or the semiconductor pixels in a CCD.

The eye aims to admit just enough light to activate the photoreceptors but without compromising the quality (acuity) of vision; but if too much light starts to enter, the photoreceptors are saturated and can no-longer pick up the difference between lighter and darker parts of an image, so everything looks white.

Light entry to the eye is controlled by making the pupil larger and smaller. But when you take drugs to dilate (open) the pupil this control is lost so the maximum amount of light flows into the eye, blinding you.

So the answer is yes, that amount of light is always present, you just exclude most of it by narrowing your pupil to a small size, except at night of course, when the pupil opens to its maximum extent to gather as much light as possible.

Chris chris, Mon, 28th Jul 2008

Thanks very much for your comprehensive answer. I thought it must be as you state in the last paragraph. I find it difficult to get to grips with the thought that on a sunny day the world is a much brighter place than we normally appreciate. Brian. Brian Kennedy, Mon, 28th Jul 2008

You're welcome Brian, good to have you with us.

C chris, Tue, 29th Jul 2008

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