Re: Why can your eye focus on objects clearer when light is eliminated

Your observation is a really neat one, although I had never heard of it 
before -- even though I've worked in optics and lasers for 27 years.

After I read your question, I went to the kitchen, got a piece of aluminum 
foil, poked a hole in it and took a look at a newspaper.  Because I'm far-
sighted (which everybody gets when they get old!), I wasn't sure what I'd 
be able to see.  What I found was that I could read the paper perfectly if 
I put it in sufficient light -- although without the pinhole, it was 
really blurry.

I thought about it for a while before I came up with an explanation.  The 
reason near- and far-sighted people have blurry vision is that their eyes 
are not perfect lenses.  Either because of genetic issues or accidents or 
aging, many folks have eyes that don't quite work as they should.  On some 
people, the cornea (the outer clear part of the eye that you see when you 
look at it) is a little flatter or more bulged out than it should be or 
maybe it's curved a little more in the up/down direction than side-to-
side.  Or maybe the muscles in their eyes don't pull evenly so that when 
they try to focus, the lenses get a little misshapen.  Or maybe they're 
like me -- at age 54, the lenses in my eyes have gotten a little stiff and 
leathery and don't change their shape enough when the focusing muscles try 
to move them.  In any case, when your eye is misshapen for whatever 
reason, the rays of light from a point on any object which enter the eye 
at different points across the pupil (which is the "black" area you see 
when you look at somebody's eye) just can't all focus at individual points 
to form a sharp image if the things you're looking at.  Edges look indistinct 
and small points look smeared out or even like lines.  The extent to which 
these things occur is governed by how badly out of shape your eye is.  

When you put on glasses, you artificially put a material (the plastic or 
glass in your glasses or contacts) in front of your eyes to introduce just 
enough pre-bending of the light rays as they go into your eye so that when 
the rays hit the imperfectly curved eye structures, their bent path causes 
them to focus nearly perfectly to points on the retina of your eye to 
produce a sharp image.  The glasses work over the entire open area of the 
pupil of your eye to "correct" the errors introduced by your internal eye 
lenses.

The pinhole achieves a similar result, but much differently.  The pinhole 
only allows a tiny portion of your eye to receive light, rather than the 
whole area corresponding to the pupil of your eye.  While your whole eye 
may be misshapen so that light entering at various points focuses at 
different points on the retina, at each point on your eye it's really 
nearly perfect in shape -- the problem came because the shapes of 
different areas across the eye were different, causing light from any 
point being observed to focus at different points on the retina.  Because 
the pinhole only allows light to hit a small area of the eye's input, it 
focuses very nicely.  Unfortunately, the fact that only a small area of 
the eye's pupil is illuminated means that the image will be quite dim 
unless care is taken to provide a very bright source with which to make 
the observation.

As a result, you can see that it's not the dim light that allows the eye 
to see sharply but that the pinhole is what does the trick.  For your 
science fair project, you might try to prove this by comparing how well a 
person sees when pinhole size is increased from small (make a hole with a 
real pin) to medium (make a hole with a pen or pencil point) to large 
(make a hole with a straw).  You could compare this with a person wearing 
one or two or three pairs of sunglasses to see if the effect is due to the 
presence of the physical aperture or the reduction of light...  I'm 
betting on the physical aperture being the key, but you should let your 
experiment be your guide.  You might, just for fun, call some 
opthalmologists or optometrists in your area and ask them what they think 
will happen -- and see if they can predict the results.

Have fun!  This is a neat area of experimentation, because it involves 
both physiology and physics -- complex, but interesting.

Thanks for the question!