Examine the lens. Notice its middle is wider than its
ends. This is the shape of a convex (or converging) lens.
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!
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 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."
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.