Light, Sound and Waves

Using the telescope like an astronomer

Practical Activity for 14-16 PRACTICAL PHYISCS

Class practical

Placing the final image at reading distance from the eye, as an astronomer would.

Apparatus and Materials

For each student or group of students

  • Telescope mount or metre rule with Plasticine or Blu-Tack
  • Retort stand and bosses, tall
  • Convex lens (+14 D), plano-convex if available
  • Convex lens (+ 2.5 D), plano convex if available
  • Greaseproof paper or frosted screen
  • Mounted lamp holder (one per class)
  • 200-watt carbon filament lamp (one per class)

Health & Safety and Technical Notes

The mains lampholder must be fitted with a suitably-fused 13 A plug. It is best if the batten holder is one of the safety pattern types, where inserting a bulb operates a switch.

Read our standard health & safety guidance


  1. Hold a page of print beside the telescope, about 25 cm from your eye. Look at the remote lamp through the telescope with one eye, while you read the print with your other eye.
  2. Concentrate hard on the naked eye, while you move the telescope eyepiece to bring the lamp into focus.

Teaching Notes

  • This is the same procedure as in the experiment Making a telescope, but try to place the final virtual image only 25 cm away - as an astronomer who wished to make sketches in a notebook would place it. The astronomer would want to move quickly from telescope to notebook without having to re-focus their eyes.
  • If a student finds this too difficult, yet wants to succeed, do the focusing for them. Let them have a good look, with both eyes open; then move the eyepiece and let them try for themselves. This works well for a student who is held back by not knowing what to look for.

This experiment was safety-tested in March 2007

can be analysed using Refraction Formation of Images
can be analysed using the quantity Focal Length Power of a Lens
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