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Help me see the light.

I'm in the middle of writing a lab exercise on the physics of the eye,
suitable for college freshman non-physics majors. I'm looking for simple
demonstrations which can be done with simple apparatus, or no apparatus.

There's a lot of literature on psychology and physiology of vision. But
less on the physical considerations.

The blind spot is easily demonstrated. Move your head closer to the screen,
about 1.5 feet from it, and look at the 1 below with the left eye, and
then see which of the letters disappear:



0        A B C D E F G H I J K L M N O P Q R S T U V W X Y Z          1



Or you can put the flashing cursor just below the line and move it from
left to right until it disappears. Now do same by looking at the 0 with
your right eye. Draw the necessary conclusions. Calculate the angular
distance of the blind spot from the center of your visual field.

Retinal inversion can be observed by holding a pinhole in front of a
bright light, a few cm from the eye, and a small object, pinhead perhaps,
near the eye, within its focal distance. The shadow image you see is not
inverted on the retina, and therefore seems inverted to your brain which has
learned to deal with inverted retinal images.

Myopia, hyperopia and astigmatism are no problem to demonstrate.

Response of the iris to bright light is easy to study, and one can show
that bright light into only one eye causes the iris of both eyes to
respond.

Veins on the retina can be seen by placing a pocket penlight gently on
the skin just below the outside of the eye, against the lower edge of the
eye-socket. Do not press hard, but move the light gently, and you see the
veins' shadows on the retina, looking like a map of the Amazon river. The
light enters through the side of the eyeball, the veins are on the front
surface of the retina (a very poor design) and cast shadows on it.
Retinal fatigue makes the shadows disappear rapidly, hence one must keep
the light moving a bit. Do not press the light hard against the eyeball.

The Pulfrich pendulum is easy to show. Nice neutral density filters can be
obtained from your optometrist (given to folks who have to go out in
bright light after having their eyes dilated). Your optometrist may be
willing to loan or give you other useful things for such a class project,
or may even be willing to give a guest lecture (be careful, though, some
of them aren't too swift on physics). Mine volunteered loan of an
ophthalmoscope and some other measuring equipment.

Hadinger's brush may be looked for.

But here's a few which elude me:

(1) The back of the eyeball isn't spherical. It isn't even symmetrical. The
lens, of course, has severe curvature of field. What simple experiment
can demonstrate these facts? [No, we will not allow a student eye to be
removed and dissected.]

(2) The eye's lens surely has spherical aberration and chromatic aberration.
Under normal conditions these are not noticed. Can we make them noticeable
by any contrived experiment?

(3) Even a normal eye, not in need of glasses, must have residual
astigmatism and coma, just as any spherical lens does. Can we demonstrate
this in any simple manner?

        -- Donald

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Dr. Donald E. Simanek                            Office: 717-893-2079
Prof. of Physics                    Internet: dsimanek@eagle.lhup.edu
Lock Haven University, Lock Haven, PA. 17745          CIS: 73147,2166
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