Tutorial details

The first section of the tutorial helps students develop a functional understanding of focal point. We have found that even after lecture instruction, many students do not recognize that the image of a distant object viewed through a converging lens is located in front of the lens at the focal point of the lens. In tutorial, students explicitly determine the location of the image of a distant object that lies on the principal axis of a converging lens. By also considering reversibility of the paths of light, students are led to develop rules for two of the principal rays: (1) A ray that arrives at a lens parallel to the principal axis passes through the focal point on the other side of the lens. (2) A ray that passes through one of the focal points emerges parallel to the principal axis.

In the next section, students apply what they have learned to determine the paths of all the rays that leave a given point on an object and pass through a lens. They are led to develop a rule for the third principal ray: a ray that passes through the center of a lens is (essentially) undeflected. Students also draw rays other than the principal rays. They do so by considering (1) a second point on an extended object, and (2) an object that lies closer to a lens than the focal point.

In the final section of the tutorial, students are asked to predict how the real image produced by a converging lens would change if certain changes are made. The practice helps students strengthen their ability to draw and interpret ray diagrams.

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Prerequisite tutorials

The Plane mirrors and Ray diagrams tutorials are prerequisites to Convex lenses.

Other prerequisites

This tutorial builds on ideas developed in the tutorials Plane mirrors and Interpretation of ray diagrams.

Equipment

List

• long-filament bulbs & sockets
• white board or large sheet of paper with markers
• convex lenses of focal length between 50 mm and 150 mm (exact value unimportant)
• small balls of modeling clay (to stand lenses upright on table)

Discussion