This activity demonstrates how to make a simple telescope using two lenses and a cardboard tube.
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.