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PRACTICE / Focal length experiment

Two sharp images on one bench

An object and a screen are fixed 0.90 m apart. A thin converging lens gives sharp images at two positions separated by 0.30 m.

Find the lens focal length.

Organize the givens & assumptions
  • D = 0.90 m
  • Lens-position separation d = 0.30 m

Paraxial thin lens in air; all distances along the optical axis; both images are real.

  1. A0.30 m0.30\ \mathrm m
  2. B0.15 m0.15\ \mathrm m
  3. C0.225 m0.225\ \mathrm m
  4. D0.20 m0.20\ \mathrm m

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