As part of the characteristics and properties of waves unit, the seventh grade students learned how to measure the focal length of a lens using an optical bench. This lab instrument allows students to manually focus a candle flame onto a screen. By measuring the distance between the lens and the screen the students are able to determine the focal length of the lens. The student compared the focal length of two lenses based on their observations concluded that it is not the overall size of the lens that determines focal length, it is the curvature that accounts for focal length. They were surprised to see that the smaller sized lens had both a shorter focal length but produced a much larger image on the screen. Students then discussed practical applications of lenses in glasses, microscopes, telescopes and magnifying glasses.
