PHO 101 Photonics Concepts
Three Rivers Community College ONLINE

Home Lab #2

The Pinhole Camera

Be sure to visit the pinhole camera web site before you begin this lab (http://www.pinhole.org/  ). The photos in the pinhole photo gallery required extremely long exposure times. You will see that the images in the pinhole viewer are very faint because so little light is captured by the pinhole. To see photographs of the pinhole viewer for this lab, and an image formed with the viewer visit: http://webpages.charter.net/laseroptics/optics/opticsindex.htm

Objectives

1. To demonstrate the rectilinear propagation of light by constructing a pinhole "camera" (viewer)

2. To apply the concept of similar triangles to a pinhole camera

Equipment

Theoretical overview

A pinhole camera consists of a closed box with a small pinhole centered on one end. As can be seen in Figure 1, rays of light from the top of the object and those from the bottom of the object do not overlap on the back of the box because of the small size of the pinhole. Thus, an image of the object is formed on the back wall of the box.

To use the box as a camera, the box is made light tight (except for the pinhole, of course) and photographic film is placed at the image location. Exposure times are extraordinarily long (up to several hours), but the oddly blurred photographs are favored by many art photographers. For direct viewing, the bottom of the box is removed so that the image is visible from below. This is a recommended method for viewing solar eclipses.

 

Figure 1

If the distance from the object to the pinhole is do and from the pinhole to the screen is di, the height of the image hi is related to the height of the object ho by similar triangles:

Procedure

Cut a hole approximately 5 cm square in the center of one end of a small carton or large shoebox. To make the pinhole, stack 5-6 pieces of aluminum foil cut slightly larger than the hole in the box (say 8 cm square). Pierce the stack with a needle. The inner foil pieces should have neat pinholes, with clean edges. Tape one of these foil pieces over the hole in the box, centering the pinhole. You can also use a piece of a soft drink can, with the hole slowly drilled first from one side, then the other. You don't want a ragged pinhole.

 

Turn off all of the lights in the room. (You won't be able to see the image unless the object is very bright and the room is very dark.) Aim the pinhole toward a light source and look up through the open bottom of the box to observe the image on the side opposite the pinhole. A lamp at the end of a darkened room makes a good object. The sun makes an excellent object too, but it is difficult to see whether the image is upright or inverted.

LAB REPORT Please write out (in complete sentences!) answers to the conclusions/observations and questions below:

Conclusions/Observations:

1. What did you use for an object?

2. Describe the image you saw- was it upright or inverted when compared to the object? Sharp or blurred?

3. Describe any other features about the image that you noticed.

Questions:

1. The blotchy shadows, dark with bright circles, under a leafy tree are actually pinhole images of the sun. How does this occur (What forms the pinholes?)? During a partial eclipse of the sun, the sun appears as a bright crescent. What will the shadows under a leafy tree look like during this phase of a solar eclipse? (If you get a chance to witness a summertime solar eclipse, be sure to look at the shadows under a large, leafy tree- it's an amazing sight!)

2. One photographer who favors pinhole cameras says he never needs a telephoto lens to obtain a larger image- he just makes a longer camera. Explain why this is so, using the equation for similar triangles given above.

 

©J Donnelly 2001