A Galilean type refracting telescope. At the far left is the convex objective lens. It has a focal length f1 , the length at which it brings light from a distant object to a focus. The concave secondary lens (focal length f2) redirects the light towards the observer’s eye, where the lens in the eye itself — often not mentioned in descriptions like this! — brings it to a focus on the observer’s retina.

Adapted from an illustration in The History of the Telescope by H.C. King, Sky Publishing Corporation, Cambridge Massachusetts, 1955, drawn by Linda Wooliever.