Make a Telescope


  • 2 convex lenses with different focal lengths
  • A card board tube that’s length is at least that of two focal lengths of the lenses when added up
  • Sticky tape
  • Color felt tip pen


  1. Take your cardboard tube and cut it down to the length of the two focal lengths of the lenses (for example: if I have 10cm and 20cm lenses, I would need a 30cm tube). Hold a lens at the floor under a ceiling light and move the lens up until you see an image of the light fixture on the floor. The distance between the floor and the lens is its focal length.
  2. Now attach the lenses to either end of the tube using the sticky tape, but try not to obscure the view through the tube.
  3. Using your felt tip pen, color the end of the tube with the shorter focal length lens so you know which direction to look through (the end with the lens with the shorter focal length).
  4. Now look though the end that you've marked and you're looking through a telescope! The first thing you'll notice is that the image you see is upside down. Real telescopes have many lenses in them so the image always appears the right side up.
  5. If the focal lengths of your lenses were about the same, you might not think there's much of a difference in image size. If you look through the telescope with one eye, however, and look straight at the object with the other you should be able to see both the magnified and the unmagnified image and compare the two. Guess how many times larger the image is than the object. In theory if you divide the larger focal length by the smaller one you should get the magnification of the telescope. If you take the case above where we had a 10cm focal length lens and a 20cm focal length lens then you divide 20 by 10 and get 2. Therefore that telescope would have a magnification of x2. Did you guess the right magnification for your telescope?


Telescopes use lenses to bend incoming light in such a way that the image appears larger.

The magnification of a telescope depends on the arrangement and the focal length of the lenses within it. The quality of the picture depends upon the size (diameter) of the lenses. The bigger, the better. You can't make good quality lenses over 1 foot in diameter so you have to use curved mirrors as lenses instead. Some of the largest telescopes have mirrors over 8 meters in diameter.