Telescope magnification

Class practical

Measuring magnification by direct comparison of object and image.

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' where inserting a bulb operates a switch.

Each scale should be vertical, with horizontal lines on it to act as an object when the magnification is estimated. A strip of shelf-paper can be used, or one or two sheets of A3 paper taped end to end. Rule thick horizontal lines on it every 10 cm, to make a coarse scale. The lines should be numbered. The experiment is easier if successive lines are drawn in different colours. 

It helps if the room is three-quarters blacked out. Since the scale is vertical, it may be viewed obliquely, so telescopes may be spread out at the other end of the room. 

Procedure

a Post the scales, high up, at each end of the room. Illuminate them with shaded lamps to prevent glaring the observers' eyes. 

b Direct the telescope at the scale, and focus it so that the final virtual image of the scale rests on the scale itself. Then, keeping both eyes open, estimate the magnification by concentrating the telescope eye on one division of the image, and seeing how many divisions of the original scale the image division covers. 

Teaching notes

1 You might suggest that students measure the focal lengths f₁ and f₂ of objective and eyepiece - just roughly by catching a window's image on an opposite wall - and see whether f₁/f₂ agrees with their estimate of magnification. 

2 The magnification will be found to be the ratio of: 
 
the focal length objective lens divided by the focal length of the eyepiece lens. 
 
The separation of the lenses will be the sum of the focal lengths f₁ + f₂. At that spacing of the lenses, the telescope is referred to as being in 'normal adjustment'. 
 

This experiment was safety-checked in February 2007

Related guidance

Classroom management in semi-darkness

About telescope lenses

Using a model telescope

Related experiments

Marking a telescope

Model of a telescope with ray streaks