Teaching Physics with the Physics Suite

Edward F. Redish

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Where's the image?

In the figure at the right is shown a thin lens (indicated by a gray rectangle) and a coordinate system. The x axis passes through the center of the lens and runs along its axis of symmetry with the positive x direction indicated by the arrowhead. The lens may be treated as of negligible thickness and has a focal length f.
The points {x,y} = {f,0} and {-f,0} are marked by black dots.

A small object is placed at the position {x,0}. For each of the four cases (i)-(iv) below, indicate whether the location of the image formed (= x') is:

  • on the positive or negative side of the axis
  • closer to the lens than the focal point or farther away.

Thus your answers should take a form such as (x' > f > 0) to indicate (the image is on the positive side of the axis and further away than the focal point) or (0> x >f) to indicate the image is on the negative side of the axis between the lens and the focal point) respectively. Note that in some cases the focal length specified is negative and in some cases it is positive.

  1. -f < x < 0
  2. f < 0 < x
  3. x < f < 0
  4. 0 < f < x

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Page last modified November 5, 2002: OP24