Science Questions

Why are faint lights brighter if you don't look at them?

Sun, 26th Nov 2006

Part of the show Repairing the Retina and Spinal Cord

Question

Jim via email asked:

A car across the street has a flashing blue security light that's very faint when you look straight at it. But when you look at it sideways, the blue light's much brighter. Why is this?

Answer

I reckon it's just down to the fact that when you want to see colour, you're reliant on a different population of photoreceptors in your eye, to those which see in the dark. In our eyes we have two different populations of light photoreceptors - in other words cells that can turn light into electrical energy that the brain can understand. In the day time, and in order to see colour, you use a group of cells called cones. Cones come in a number of different flavours, and they see colour. So in other words, when light comes in, (remember light is a mixture of all the different colours), the cones respond selectively to certain wavelengths of light. The light across the road is a flashing blue light, he specifically says that. This means that it might be at a wavelength which he needs his blue cones to be able to see. Now, when we actually look at things very very closely and fixate on them, you're using the part of the retina referred to as the fovea which is where there's the greatest concentration of photo receptors, that's why your vision there is very acute. But also, that's where all the cones are, and where colour vision is, mostly. Now during the night time, and also when you look in your peripheral vision, there are fewer cones in the periphery, and you mainly use what are called rods. Now rods are much more sensitive to light and that's why they're only used in the night time. But they don't tend to be able to decode colours. So when you look at this light out of the side of your eye, you're probably seeing it using rods that are helping some of the blue cones a little tiny bit, because we know that rods can help cones a bit under low light conditions. When you look straight at it, there are fewer of these very sensitive cells and more of the less light sensitive colour specific cells so I think that's probably why you can see things more acutely in the dark, out of the periphery of your vision when they're under low light conditions, but you can read things much more accurately in the centre of your vision.

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