Teaching Physics with the Physics Suite

Edward F. Redish

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The half lens

A projector has an arrangement of lenses as shown in the figure below. A bulb illuminates and object (a slide) and the light then passes through a lens that creates an image on a distant screen as shown.

When a cardboard is brought up to cover the lower half of the lens, what happens to the image on the screen?

  1. The top half of the image disappears.
  2. The bottom half of the image disappears.
  3. The image remains but is weaker (not as bright).
  4. The image remains unchanged.
  5. The bottom half of the image becomes weaker, the top is unchanged.
  6. The top half of the image becomes weaker, the bottom is unchanged.
  7. Something else happens. (Tell what it is.)
Explain your reasoning, drawing whatever rays are needed to make your point clear

Note to the instructor: This is a classic problem. Most people (not just students) expect that you will only get half of the image.

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Page last modified October 31, 2002: OP18