Is the world much brighter than we see?

17 August 2008

Question

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 can't actually see anyway as it's in the ultraviolet or infrared ends of the spectrum. We'll 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 you're 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 isn't just the time it takes for your pupils to dilate. It's actually something to do with what's 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 can't convert it back very quickly. When you're sitting outside in a bright light it's getting converted really quickly and it's really quickly running out so when you go into the dark there's 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 Brian's question is that you are seeing slightly less light than the world produces but it's because your eyes and your brain have evolved that way to get the best out of them.

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