Hi Roger Haar!
I have really been enjoying the responses to your original
email about pinhole cameras and their theory, especially that of
William Beaty. He always has some of the most useful info that I
feel is on the level that I can use in my classes.There is a demo
that I saw Paul Hewitt do with using the shadow of a light image
that knocked my socks off one time in SFO at an AAPT meeting. I
had mentally lost all recollection of how it was done but William's
note on "light image" shadows has brought it all back! --I think! I
will have to try it out to be sure.
Anyway, in regards to the pin hole camera, I have a
suggestion for a demo that shows exactly what the light rays are
doing. This all should be in a dark room, of course. Start with a
bright object , preferably one that is flat and includes a bright point
source. The ideal would be a transparency, at least page size,
brightly lit with an additional bright point source in front of it in
contact with it (so it would be in the same plane). In front of it
place a large opaque sheet with a small hole in it, the "pinhole". A
larger pinhole will make it easier to see the light rays and the image
but it will be more out-of-focus. Behind the opaque sheet place a
white material as a screen. When the image (a picture that looks
like the original ) is formed on the screen, punch several small
peep holes in the screen. One or more in some bright areas and
one or more in the dark ones. If there are different colored areas
put some in the different colored regions. Now the students should
look from behind the screen through each of those punched holes
and look at the "pinhole". Where the punched hole is punched in
a bright area they will see light coming through the "pinhole" and
where the punched hole is in a dark part of the image they can
look at the "pinhole" and see no light. Also if there are colors in
some sections of the image, the student can peek from behind
through a whole in the screen where, for instance) there is blue
image and see that those points on the screen "look" through the
"pinhole" and see the blue parts of the original bright image. This
is possible because of the straight-line aleignment between
1)those points on the screen where blue image is present, 2)the
"pinhole", and 3)the actual blue parts on the bright object. The
actual path that the light rays take should be clear since the
students saw them individually with their own eyes.
Next place another large opaque sheet in front of the bright
image (with the bright point source in contact with it), this time with
a large (as is obtainable) convex lens. Again, place a large white
screen behind the lens. Of course this time the screen has to be
adjusted for the image to appear sharp. The image is much
brighter this time and the reason will shortly be evident. Again, look
from behind the screen through the little peep holes located at
different points in the screens image. This time the student will not
see a very faint few rays coming through a "pinhole" but many
thousands coming from the entire area of the lens. It is obvious at
this point why the image with a lens is much brighter, thousands
times brighter, because thousands of single rays are used for every
single point on the image rather than a very few. The students
have seen them with their own eyes.
The paths of these thousands of rays from every single point
on the original light object can be shown to be arranged in a
conical shape by doing the following. Next, cover all of the object
up with another opaque screen except where the point light
source is located. This assures that only the light from that
particular part is allowed to pass to the lens and the white point
source is the only part of the object that shows up on the screen.
Assuming that the image is still in focus, move the screen toward
the lens and the point of light will become a solid circle of light that
gets larger and dimmer as it nears the lens. It will be the same size
as the lens just before it touches it. This succession of circles
shows the conical shaped path that all of the rays take for those
that make up the bright point source but the process can be
applied to every single point on the object. The conical shape of
the rays in front of the lens can be verified by placing the screen
back where the circle is still fairly large and then interfering with the
rays in front of the lens. A heart-shaped hole placed in front of the
lens (and smaller than same) will allow a heart-shaped group of
rays pass and this will be evident in the previous circular-shaped
pattern becoming heartshaped. This is a secret of wedding
photographers. A heartshaped hole placed in front of the camera
lens will produce heart shaped out-of-focus images of candle light
(point sources) behind a bride and groom adding to the
atmosphere (and price) of a wedding photo. ( It must also be
understood that some of the light from the entire scene is
diminished when this id done and must be compensated for with
more time, etc.) It is also possible to show that each of these
individual rays have equal value in the final formation of the unique
point of the image (the replica of a single point on the original
object) by interfering with different amounts and areas of the cone
of rays. Different shaped objects can change the shape of the ray
group that makes up the "out-of-focus" image but when the
screen is placed back in the "in focus" position, whatever
remaining rays were allowed to pass will still make up the point just
like before. The only difference will be that the brightness will be
determined by the number of rays allowed to pass to make up the
final image. The students can once more look through the peep
hole in the screen (at the in-focus position) to see the light coming
through the lense again. This time, when part of the lens is
blocked, the reduction in the area of the cone shaped group of
light rays is seen with their very own eyes. It is no wonder that the
point is dimmer. They can then make the lens "camera" to be like
a pinhole camera with all but the very center of the lense blocked
out. This is also why a camera "shut down" acts like a pinhole
camera. A camera "shut down"is one that has the iris or the
aparture practically closed. Almost everything is in focus in this
state. It acts mostly like a pinhole camera where every image is "in
focus" because there is a simple in-line relationship between each
object point, the pinhole (the small apartrure), and the point making
up the image on the screen (or the film) which the ray forms.
Actually true focusing involves the bending and converging of
many rays to form a brighter image. The mechanics of the pinhole
camera which we could see previously involved only light that
happens to go in straight lines. Thus, focusing is not a part of
pinhole cameras and that is why they do not even have a plane of
focus which using a lens produces. The focusing process of rays
from an object by a lens involves gathering many diverging rays
and converging them into individual bright points or bits of the
image.
I hope I haven't taken up too many of your bits and bytes in
your email memory but my building is closing and your are being
saved from any further circumlocution. I hope my little dangerous
bit of experiences with lenses and photography might be of some
help to others also working with concept building of their students.
Goodnight!
Gordon Shepherd
Guilford Tech. Comm. Coll.
Jamerstown, NC 27282