Next, ask them to put a pen or pencil into the beaker, halfway out of the water, and then view it again from the side. The pencil will seem broken, as if the half below the water were unconnected to the half above it. Ask students to speculate about the reasons behind these optical illusions. As light travels through a convex lens, its waves are refracted toward a single focal point. The thickness of the lens determines where this focal point lies.
Thick lenses have shorter focal lengths than thin lenses.
Have students hold a flashlight 2 meters from a comb, which itself should be held perpendicular to a piece of white paper. Students should first trace the shadows cast by the comb. When the shadows match, the group should measure the distance between the lens and the comb to determine its focal length. This process can be repeated for each subsequent lens. When their focal length data are complete, ask students to determine the relationship between lens thickness and focal length.