Teaching Notes

If you have neither a laser nor a light box, and you must use a flashlight for a light source, you can still encourage the students to get the general idea of how parallel light beams are reflected from curved mirrors (toward the mirror axis or away, as discussed in Background Information). The students can still see images made by a light bulb in the second half of the activity. When the students observe images, give them the best mirrors you have available.

Once the students have observed the images, you may want to remind them about Steps 1 to 7, when they observed reflections of individual light beams. Encourage them to use their observations to predict which mirrors make real images (as discussed in Background Information above).

You may want to set up a light bulb in the front of the room where it is visible to all groups. This bulb can serve as the distant object, to minimize how the students must move about the lab. If your students use lasers, emphasize the importance of keeping their laser beam within their work area.

If students use a laser, have each group prop up a large piece of cardboard to absorb the beam. If the students use a 110-v bulb as a light source, caution them to handle the bulb carefully and to leave the bulb in its socket at all times. Be sure there are no exposed connections to the socket. If the socket is ceramic and has screw-terminal posts underneath, screw the socket to a piece of wood so the terminals are inaccessible.

Many students believe that the virtual image is located inside the mirror. Many others believe that the image of an object moves if the observer moves. To help students, you can have them set the mirror on a table and place a small object nearby. Have them observe the reflection of the object from different positions and report what they see. You might also have them repeat this observation with a plane mirror.

This data was taken with a cosmetics mirror.

In the above data, note how in going from 5.13 m to 26.5 m, more than a factor of five in object distance, the image moved in less than fifteen percent. The focal length of the mirror is very close to .37 m. Also notice how the image distance increases rapidly as the object approaches the focus of the mirror.