Making a light beam

In this section we will learn how to set up a light beam station. This will be a tool that we will use in the following sections, as we study the properties of mirrors and lenses. A light beam is the opposite of a shadow, and so you can't understand one without understanding the other.

The things that we will need are

The file folder will be used standing up, as shown in the pictures. Cut a slot in the folder: it should be vertical when the folder is in place, 1/8" to 1/4" wide, with straight and parallel sides; it should be almost as tall as the filament of the light bulb in the lamp base. Tape another folder to the first, to make an enclosure around three sides of the lamp (and so that the folder isn't falling down all the time). Turn off the room lights, turn on the lamp, and you get your beam!
Here are some things to observe:

  1. Turn the lamp to left and right, and notice the effect on the beam. Does the orientation of the lamp matter? If it does, can you see what property of the bulb is involved (look carefully at the bulb when it is turned off)?
  2. Try placing the lamp closer and farther from the folder with the slot in it. What effect does this have on the beam?
  3. Compare the beams made using clear and frosted light bulbs.
  4. Suppose the slot was not cut as high as was recommended. What effect does this have on the beam? Tape a card over the top 10 cm of the slot to verify your prediction. Notice what happens with the too-short slot when you move the lamp close to the folder with the slot in it.
  5. Change the height of the bulb above the tabletop to examine the effect.
  6. What would happen if the slot were not straight? Make a slot that weaves back and forth by about an inch. Does the result contradict our belief that light always travels in straight lines?
Draw a sketch of the set up of your system that produces the best beam. Include measurements so that you can easily set it up again.

All done?   On to an activity!