I do Physics A level and in my last exam it was really
annoying because one of the questions was something that I've always
wanted to know but never got around to finding out.
You know when you look at a star in the sky or those glowing stars that
you stick on your bedroom ceiling? Well, why is it that they don't look as
bright when you look directly at them, but do when you look slightly to
The answer is to do with the biology of the eye. There are two types of
receptor cells in the eye and each has strengths and weaknesses:
1. Rod cells - only see black and white but are very sensitive (ie will
pick up very small amounts of light)
2. Cone cells - see color but are not as sensitive as rod cells.
At the point in the back of your eye (the retina) where your eye focuses
the light when you are looking directly at something there is an area
called the fovea. In the fovea the cells are very densely packed so that we
can see really well and because color is important to us the cells are
predominantly cone cells and only a very few rod cells. The problem with
this is that when the light is dim - the cone cells in the fovea are
actually really bad at picking up anything at all.
Outside the fovea there are more rod cells. Consequently in low light you
are better off not looking directly at an object because then the light
will fall on the (predominantly) rod cells outside the fovea rather than
the (predominantly) cone cells inside the fovea.
Hope you did OK in your exam anyway.
That's not physics, that's biology. The reason is that when you look at
something, its image falls on a region of your retina called the fovea. In
the fovea, there are no rod cells, only cones. Cone cells give color
recognition and better resolution than rod cells, but they aren't as
sensitive to light as the black-and-white only rod cells. When you look
slightly to the side, your more sensitive rod cells can detect the
starlight, so you can see it better.
Richard E. Barrans Jr., Ph.D.
Your retina has two types of sensory receptors: rods and cones. The cones
are for color vision and operate best during the day when light is shining.
They are located in the middle of your retina because most of what you need
to see is directly in front of you. At night, the rods allow you to see in
the dark, or to see shades of gray and black. They are located at the
periphery of your retina. At night, when you try to see something in the
dark, if you look directly at it, the cones are not stimulated in the dark.
If you look to the side, or in your peripheral vision, the rods are
stimulated and you can see the object more clearly.
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Update: June 2012