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
A dimly lit room
A lamp base with a clear light bulb
Something from which to construct screens. Ordinary
file folders (preferably dark-colored ones) are useful
for this -- they are easy to cut, and already have a
fold in them, which makes it easy to get them to stand
up.
A sheet of white paper, which will be the viewing screen
on which the light beam will be viewed.
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:
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)?
Try placing the lamp closer and farther from the folder
with the slot in it. What effect does this have on the
beam?
Compare the beams made using clear and frosted light bulbs.
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.
Change the height of the bulb above the tabletop
to examine the effect.
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.
