[Brknndy@aol.com]

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?

[chris]

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

[Brian Kennedy]

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.

[chris]

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

C