Demos: 7A-34 Image by Partial LensTo illustrate that any portion of a lens can produce a complete image, an opaque card blocks more and more of the lens until it is completely blocked. It is only when the lens is completely covered that the image disappears. And the image remains complete (but dimmer) until it disappears.
Directions: Dim the room lights and use a large diameter convex lens to project an image of a frosted light bulb on the screen. (Place a blocking device behind the bulb to prevent glare in the students eyes.) Once the image has been focused on the screen, slowly lower the card in front of the lens. The image will get dimmer, but remain complete until you have completely blocked the lens.
Suggestions for Presentation: Common sense tells the student that if you block a portion of the lens, you will lose the corresponding part of the image. Since the image is upside down, ask the students, If I lower this card in front of the lens from the top down, which part of the image will be blocked first? After gathering the responses, slowly lower the card. There will be considerable surprise that the image remains whole. This presents an opportunity to use ray diagrams to show that light from all parts of the bulb can still reach the screen as long as there is some part of the lens exposed. Point out that if you were to mask off the outer portion of the lens, you would just have a smaller lens, but you would still get a complete image, just dimmer.
Applications: The iris diaphragm on a camera allows more or less of the lens to be used. If the diaphragm is opened all the way, more light gets in (low f-number); if stopped down (high f-number), less light gets in. (Why one changes the f-number has to do with depth of field and requires examinations of other lens properties.)