Teaching Notes

Most students need help when they first make a real image with a lens. They often have difficulty getting started because they are unable to line up the lens, the file card, and the object (with the lens facing the object). To help them, you can refer them to the drawing in the Student Book on page 237. Projecting the slide (Step 9) can be challenging. Many students make an image of the light bulb as seen through the arrow, but it is in fact the bright arrow that is the image. To enlarge the size of the image, they must bring the lens and arrow closer together. You may need to help them move both lens and arrow to find the position that gives a sharp image.

Keep the groups as well-separated as possible. Even with the room dark, stray light from one group’s bulb can easily interfere with other group’s images.

If the students use a 110-V light bulb for the light source, be sure the students cannot touch any exposed wiring. If you use ceramic sockets with exposed screw terminals underneath, screw down the socket to a board so the terminals are inaccessible. See Advance Preparation and Setup. If the students use a candle, be sure no flammable materials are nearby.

Light is a challenging topic for students. Students do not easily understand the fanning out of reflected light nor, for that matter, even the idea that light travels from one place to another yet is not material. You can help your students by asking them to explain how they hold the lens when they make an image (facing the object) and how that orientation produces an image (the lens focuses the light that hits its surface, and the light, which fans out from the object, hits all of the surface).

Focusing Parallel Beams: If you have lasers available, set up the lens and laser so you can move the laser back and forth in front of the lens, with the beam parallel to the lens axis. Observe what happens to the beam after it passes through the lens. Measure the focal length by making an image of a distant object and compare the value with what you found from the laser beam.