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Convex and concave lenses


In an optics experiment, Isaac places an apple 36 cm from a concave lens. He looks through the lens to see a virtual image that's one-fourth the size of the apple.
What is the focal length, f, of the lens?
Note: Let's write the answer in the Cartesian sign convention.
  • Your answer should be
  • an integer, like 6
  • a simplified proper fraction, like 3/5
  • a simplified improper fraction, like 7/4
  • a mixed number, like 1 3/4
  • an exact decimal, like 0.75
  • a multiple of pi, like 12 pi or 2/3 pi