A telescope can be as simple as a cardboard tube and two pieces of glass. But how does such a telescope make objects appear closer than they really are? Understanding this is key to understanding how a telescope works.
In constructing a basic telescope, the two pieces of glass used are magnifying lenses. The cardboard tube is only there to hold the two pieces of glass at the right distance from each other. When you hold one end of the tube to your eye and the other toward the object you want to appear bigger, the piece of glass away from you collects a lot more light than your own, smaller eye can. When the light enters the piece of glass, the glass collects as much light as its size allows, and then bends the light into a single point. This light continues towards your eye, and the piece of glass at this end makes the light the size of your eye's pupil.
Think of the light being in a giant funnel. The funnel can hold as much as it's size allows for. The funnel passes its contents through a narrow opening, and this is collected at the other end in a shape determined by the dimensions of the container.
There are two major classifications of telescopes: the refracting telescope, and the reflecting telescope. In its simplest form, the refracting telescope follows the simple paradigm described above. The lens turned skyward is called an objective lens. The eye-end lens is encased in an eyepiece, which may be at an angle for comfort. Refracting telescopes produce the sharpest images.
A reflecting mirror adds an additional segment to the route the light takes before it reaches the eye. The eyepiece is mounted on the side of the telescope, near the end by the viewer. When the light enters, instead of going through a lens at the other end, it is reflected by the objective mirror, which reflects the light back towards the end where it enters. Just before it reaches the very end where it entered, it is reflected by a secondary mirror, which sends it at an angle out through the eyepiece. Think of bouncing a ball against a backboard, and then right before it gets back to you someone sticks up their hand and deflects it to the side.
This is the most basic type of reflecting telescope. There are more complicated and sophisticated versions of reflecting telescopes which have additional types of lenses and mirrors, causing the light to follow an increasingly complex path. Depending on the modification, they can allow such things as making a telescope smaller without compromising the quality of the image, and changing the orientation of the reflection of the object.
Though a telescope can be as simple as a pair of binoculars, or as complex as the Hubble Space Telescope, they all work on the basic principles of collecting and "bending" light. By their ability to gather more light than your eye can, and then magnify the image of an object, they are able to make objects appear to be brighter and larger.
The Illustrated Encyclopedia of the Universe Ian Ridpath, editor. Watson-Guptill Publications. NY, NY. Copyright 2001 The Foundry Creative Media Company Ltd.