In this section we will meet the light 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.

This device consists of

• The light bulb, which has been specially designed for the light station.
• A base with a socket for the light bulb. The socket is mounted so that it can be rotated by turning the wheel at the bottom of the base.
• Side walls. The front opening has a slot that will hold the various masks that will be used (the photograph shows the single-slot mask, for example).
• Lid
• Transformer

Assemble the light station, insert the single slot mask, turn off the room lights, turn on the lamp, and you get your beam! The beam will be more visible on a white surface, and it helps to turn off some of the lights. Total darkness is not necessary.
Here are some things to observe:

1. Turn the wheel on the base 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. The slot in the slot mask has a certain shape and size. In particular, it ends before it gets to the top of the slot mask. How would the performance of the light station change if the slot had been cut higher, or not as high? You can test your answers to these questions by turning the slot mask over (so that the slot goes higher and by covering the top 2 cm of the slot (so that it doesn't go as high).
3. What would happen if the slot were not straight, and instead wove back and forth a centimeter or two? You can test your explanation by cutting a slot in an index card, and using this as a temporary slot mask. Does the result contradict our belief that light always travels in straight lines?

All done?