Originally Posted by
ronhip
A really simple experiment is to stand a bit back (maybe a dozen feet) from the expanded spot on the wall. Stand far enough back so the speckle is obvious. Then slowly move your head left and right. If the speckle moves in the opposite direction to your head, you're nearsighted. It the speckle moves the same direction, you're farsighted. If it also moves somewhat diagonally, you have some astigmatism.
The speckle happens IN YOUR EYE, not on the wall. You can prove this to yourself by making a pinhole in front of your eye with your fingers while you look at the speckle. The smaller the pinhole, the bigger the speckles become. Now, there's no way a pinhole can change the size of the dots if they were on the wall, so it must be happening in your eye. The speckle is light interfering on your retina. Light coming through one part of your iris, interferes with light coming through other parts of your iris.
The things you learn working at a science museum...
Pretty cool!