I can't help but smile, broadly, when I think of the amazement that my students have when they see, first hand, how a box with a hole in it can produce a permanent image, especially an image that they are in - a "class portrait." I pull this stunt in every photography class I teach at the College of DuPage. It goes like this:
Using a powerpoint presentation with all kinds of diagrams and pictures, I explain the phenomenon of scattered light collecting in an organized way (being focused) as it travels through a hole in one wall to form an image on another wall; the smaller the hole the more organized the light becomes, leading to a sharper image.
I then show my class a cardboard box with a hole in it. It's about 8x8x10 inches (I made it myself in 1983). "Inside," I say, "I've placed a piece of photographic film against the back wall. It will react to the light coming in through the pin hole in this piece of aluminum foil. It's the only way that light can get into the closed box. Right now, the pin hole is covered by a piece of cardboard - it's the shutter. The film will become completely black wherever the greatest amount of light hits it. Wherever light doesn't hit the film at all, the film will remain white. At all the levels of light in between, the film will react proportionately to the amount of light striking it producing gray tones. THUS," (I love that word! It's so Sherlock Holmes, and, if I say it loud enough, it wakes up the students that I've put to sleep with my lecture.) "THUS, the image we pull out of the box will be in reverse tonality; it will be a negative. The bright sky will be black and Jamie's black hoodie will be white."
With great anticipation (at least on my part) I lead them outside and I get them to pose in front of the box for an exposure that can be anywhere from 12 seconds to 60 seconds, depending on cloud conditions. I open the shutter, watch the stopwatch... 3, 2, 1... and close the shutter. After the exposure we go into the darkroom, the camera obscura. The film goes into a tray of developer solution and s l o w l y the image is revealed! My students are impressed. They can see detail. "Cool," they say, as they go on a short break. Now, I prepare for the best part.
I finish washing and drying the negative and I set it up on a scanner. The class resumes and the scanner buzzes and hums and displays the negative on a computer screen, in glorious black and white, and upside-down. I explain that, "this is how the camera saw you guys." -shown above. (By now they've figured out that the box with a hole in it is a camera.) I explain again, as I did in the powerpoint presentation, that the image in the back of the camera was formed by light traveling in a straight line. Light from the sky above traveled through the hole to the bottom of the camera; while the light reflecting off Jamie, standing on the far right, traveled to hit the far left of the camera, and the light bouncing off Kelly, standing on the far left, hit the far right of the camera. So, not only is our negative reversed black to white, it is also reversed left to right, and up to down.
I then hit a button and the negative is rotated right-side-up. It is still reversed left to right.
Then the real amazement sets in when I push a button that turns the negative into a positive.
I love getting all the credit for something that's been done for over 180 years!