Focal length of concave lens experiment

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SUMMARY

The discussion focuses on methods to determine the rough focal length of a concave lens and a convex mirror without using a convex lens. Participants suggest using sunlight or an overhead light source, which approximates an infinite distance, to measure the focal length of a convex lens directly. For concave lenses, the challenge lies in the virtual image; however, by taking measurements at different object distances, one can use geometric principles to estimate the virtual focal point. This method provides a practical approach for students during practical exams.

PREREQUISITES
  • Understanding of basic optics principles, including focal length and lens types.
  • Familiarity with geometric optics and virtual images.
  • Ability to perform measurements accurately with minimal parallax error.
  • Knowledge of using light sources for optical experiments.
NEXT STEPS
  • Research methods for measuring focal lengths of concave lenses using geometric optics.
  • Explore the principles of virtual images in concave lenses and mirrors.
  • Learn about practical experiments involving light sources to determine focal lengths.
  • Investigate the use of different types of lenses and mirrors in optical setups.
USEFUL FOR

Students in physics, educators teaching optics, and anyone conducting experiments related to lens properties and measurements.

ash_nar
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Is there any way to find rough focal length of concave lens and convex mirror without using convex lens? Though I did the experiments(with object and image needles, not candle) very carefully with no parallax errors during my practical class, my readings are incorrect. I want to know if der s way to find out rough focal lengths of diverging lens n mirror in a simple way so that I can manipulate my readings accordingly during practical exam... Plzzzzzz help...
 
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One easy way to get a rough estimate is to use either the sun or an overhead light as the source- the souce is then approximately at infinity. For a convex lens, the measurement is the straightforward- the distance from the lens to the focused spot is the focal length. For a concave lens, it's a little more difficult because the image is virtual. However, by making two measurements at different distances from the lens to calculate the divergence, you could calculate where the virtual spot is by geometry.

At least that's what I would do for a first approximation.
 

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