University of Virginia
Physics Department

Convex Lenses

A Physical Science Activity

Student Activity

Materials

• Magnifying glass (or other convex lens)
• Large window
• 2-pieces of paper (one lined, one unlined)
• Meterstick

Procedure

1. Examine the lens. Notice its middle is wider than its ends. This is the shape of a convex (or converging) lens.
2. Measure the focal length of the lens. This is best done using the sun as the light source (the sun is so far away, its light rays reach the earth essentially parallel.) Stand with your back to the sun. Hold the lens so the sun shines through it and onto a sheet of paper. Move the lens towards and away from the paper until the dot of light is focused. The distance from the center of the lens to the surface of the paper is the focal length. (Holding the lens in this position long enough can actually ignite the paper.) DO NOT PLACE YOUR HAND IN THE PATH OF THE LIGHT OUT OF THE LENS. DO NOT LOOK AT THE SUN WITH THE LENS!
3. Use the lens as a magnifying glass. Objects that are closer than the focal point will appear as magnified images through the lens. Try this by holding the lens close to the lined piece of paper. Look at the paper through the lens and notice that the spaces appear much larger. Move the lens away from the paper until the image is still focused but magnified as much as possible. Determine the magnifying power of the lens by counting the spaces between the lines that fit into one field of view (figure 1).

 Figure 6: Determine the magnifying power of a lens by counting the number of lines that fit in one field of view. In this example, the magnifying power is "4x."

4. Use the lens to project a real image. Objects that are farther than the focal point will appear as real images that can be projected on a screen. Stand with your back to a bright window. Dimming the lights in the room may also help. Hold the unlined paper in front of you in one hand, and hold the lens in front of you with the other. Move the lens towards and away from the paper until the image of the window behind you appears on the paper. Notice that this image is smaller and upside down.