Take three different pictures of the same object using a different-sized pinhole each time. When their photographic experiments are complete and their film has been developed, ask students to determine what aspects of human vision their photographs have revealed. The photographs from the first experiment should illustrate how images need to focus directly on the retina to achieve clear vision. 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.
They can achieve this end by holding a flashlight two 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 whether they see any relationship between lens thickness and focal length.