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LAB: Telescope Lenses

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Astronomy Lab Instructions

Astronomy Lab: Lenses

Lab Goal: Determine the focal length of a lens


Procedure for EACH lens

  1. Set up the light source so that it falls on the interesting object.
  2. Set up the stiff paper to form a fixed vertical screen.
  3. Move the lens back and forth between the object and the screen until an image of the object is focussed on the screen.
  4. Measure the object distance O (from the object to the lens).
  5. Measure the image distance I(from the screen to the lens).
  6. Determine the focal length f = O * I / (O+I).
  7. Move the screen and repeat the measurements at least three times.
  8. Take the average of the four focal length measurements.
  9. Using the lenses with the shortest and longest focal lengths, create a telescope and determine its magnification.

    You don't need to make a formal telescope. You can make cardboard stands for the lenses and set them up in line so that you can look through them at some distance obejct, and bring it into focus.

    • Which lens should be the objective (nearest the distant object)?
    • Which lens should be the eyepiece (nearest the observer)?
    • How far apart should they be to create magnification?
    • What happens to the image as you change the distance between the objective and eyepiece?


  1. A list of the materials you actually used.
  2. A brief description of your procedure, with notes on any major changes you made to the suggested procedure above.
  3. The basic measurements you observed, and the calculated values for the focal lengths of your lenses.
  4. A sample calculation.