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?
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.
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
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.