How to Find Focal Length With a Parabolic Mirror

Written by lyle berg
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How to Find Focal Length With a Parabolic Mirror
(parabolic antenna image by citylights from

Parabolic mirrors are used in astronomical telescopes because parabolic curves precisely focus light from distant objects. Light rays striking a parabolic mirror focus at a point in front of the mirror called the focal point. The distance from the centre of the mirror to the focal point is the focal length. It is important to know the focal length, because it is a factor used to calculate the magnifying power of a parabolic mirror.

Skill level:

Things you need

  • Parabolic mirror
  • Light source (lamp or flashlight)
  • Tape measure
  • Pillow
  • A piece of cardboard about 1 foot long and 1 inch wide
  • Table or workbench

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  1. 1

    Prop the parabolic mirror perpendicular to the table or workbench with the pillow.

  2. 2

    Place the light source about ten feet away from and in front of the parabolic mirror, slightly off to one side.

  3. 3

    Hold the cardboard in front of the mirror about halfway to the light source.

  4. 4

    Position the cardboard strip so that the image of the light source is reflected onto the cardboard.

  5. 5

    Move the cardboard toward and away from the parabolic mirror until the image of the light source is sharply in focus. This is the focal point.

  6. 6

    Use the tape measure to measure the distance from the centre of the mirror to the focal point. This is the focal length.

Tips and warnings

  • Telescope mirrors are made to a specific focal ratio, for instance f/8. If you know the focal ratio of a mirror, you can calculate its focal length by multiplying the numeral in the focal ratio by the diameter of the mirror. For example, a 6-inch mirror with a focal ratio of f/8 would have a focal length of 48 inches.
  • Do not use the sun as a light source for a mirror with a reflective coating. The concentrated sunlight can damage your eyes.

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