Using the telescope like an astronomer
Placing the final image at reading distance from the eye, as an astronomer would.
Apparatus and materials
For each student or group of students
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.
a 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.
b Concentrate hard on the naked eye, while you move the telescope eyepiece to bring the lamp into focus.
1 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.
2 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-checked in March 2007