Simple Telescope


For a simple two lens refractor telescope, the angular magnification is the ratio of the focal length of the objective lens to the focal length of the eyepiece. The lenses are placed a distance apart equal to the sum of their focal lengths and are then adjusted slightly as needed to focus.



Principles Illustrated

Telescopes and angular magnification

NCEA & Science Curriculum

PHYS 2.3, SCI 2.9


Telescope: have students make the telescopes, setting the distance between the lenses to the sum of the focal lengths. Something like 15 cm and 7.5 cm will be suitable, with the eyepiece having the shorter focal length. In this example we would then have an angular magnification of two. Have students look through the telescope at a clock or some other familiar device with both eye open With a bit or practice they will see two clocks, one magnified and one not.

Angular magnification: ask students which is bigger, Jupiter or a photo of Jupiter taken through a telescope? Obviously Jupiter is bigger, so why does the photo look bigger? In other words, why can we see more detail in the photo? This can lead to a discussion of how telescopes make an image that is smaller than the object but much closer, giving it a bigger angular size, and making more detail visible.


Do not look directly at the sun nor any other bright light source with the telescope, and be careful about poking eyes with the metre stick.

Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.

Related Resources

Human eye resolution

Teaching Resources

Would you like to contribute lesson suggestions? Contact us.


This teaching resource was developed with support from

The MacDiarmid Institute
Faculty of Science, Victoria University of Wellington
School of Chemical and Physical Sciences, Victoria University of Wellington


Copyright and fair use statement