Demos: 7A-35 Fresnel Lens

That a lens is just the “limiting surface” of a large number of prisms with decreasing thickness can be illustrated with a converging Fresnel lens, consisting concentric grooves in a plastic sheet, each groove having a slightly larger “slope” as one moves to the outer edge.

Directions: Show the overhead transparency(ies) illustrating the principle behind the Fresnel lens. Pick up the converging Fresnel lens and hold it near your face, facing the audience. Your face will be greatly magnified. If a negative lens is available, hold it up to show the reduced virtual image.

Suggestions for Presentation: Spend some time going over the concept of “lens as prisms.” The overhead transparency is really helpful here. Especially point out that much of the material of the lens is not needed, just the angled faces. Because the lens on the overhead projector is a Fresnel lens, refocus the projector so that the lens is focused on the screen. The concentric grooves will be evident. Use a white sheet of paper and hold it flat near the stage of the projector. Begin to lift the paper, slanting it slightly forward to show how the light converges toward the projecting lens housing. Thus the Fresnel lens serves to focus the light so that all of it goes through the optical system, not dispersed into the room.

The negative fresnel lens is grooved opposite to the positive one. You can show how it produces a virtual, erect, smaller image.

Applications: Overhead projectors, rear lenses in RV’s.

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