The telescope does not magnify objects but brings them nearer by using the principle of refraction through two or more lenses.

In the refracting telescope the first lens is placed at the wide end of the telescope and is convex in shape. This is called the objective lens, which brings the image closer but turns it upside down.

The light rays from this image reach the second lens which is known as the eyepiece lens. In Galileo’s telescope this was concave, but in 1611 Kepler suggested a convex eyepiece and this came into general use in the middle of the seventeenth century. The shape of the eyepiece lens refracts or pushes back the light rays so that the image now turns the right way up.

The power of a telescope depends on the curvature of its lenses. To focus the instrument the objective lens and the eyepiece lens must be moved in relation to each other. For this reason a telescope is made of two or more tubular sections which can slide within one another. In binoculars the focus is worked through a knurled nut which is turned.