Position yourself as far away as possible from a bright window in a room. An ideal window will have the sun shining behind a tree or other object, creating a shadow. The sun is an ideal light source because its distance from the earth creates nearly parallel light rays that will bend properly when they go through the convex lens. This makes finding the focal point of any convex lens is very simple.
Secure a piece of white paper to the nearest wall opposite the window using tape. Position the paper in the brightest area on the wall, at the same height as the window. If this position is too high to reach, use a different window or try a different time of day.
Hold a convex lens two inches in front of the paper that is taped to the wall. Move the lens slowly away from the paper. Make sure you are moving the lens in a straight line away from the paper as this will ensure a proper measurement. As the lens approaches the focal length, a clear image of the window will appear on the white paper.
Measure the distance from the center of the lens to the paper with a ruler when a clear image of the window is visible on the paper. This distance is the focal length and the focal point lies one focal length away from the lens. This process can be repeated with the opposite side of the lens to find the focal point of the other side.