Refracting telescopes were invented in the 1600s. Galileo Galilei was the first to put them to scientific use when he turned his telescope towards the heavens. Refracting telescopes utilize a lens in order to make far away objects appear closer. The lens is ground so that the edges are thinner than the middle. The lens then can be said to be convex. When light passes through the objective lens, it is focused upon a certain point inside the tube. An eyepiece is then used to view the image.
The eyepiece is very important, for this is where the magnification generally occurs. Interestingly enough, when the light goes through the objective lens it turns the image upside down. In a very large astronomical telescope this is not generally a problem, but for telescopes used to view things on Earth this can be disconcerting. For this reason the eyepiece generally contains another convex lens to turn the image right side up again.
Interestingly enough, our own eye works in a similar way, the image presented to the back of the eye is actually upside down. When our brain processes the image it turns it back right-side-up. Refracting telescopes in modern day are usually used for terrestrial purposes, bird watching, on military hardware and even hunter's gunsites.
Refracting telescopes are often given designations based on the focal length and the refracting lens. For example on a 700 x 60mm Refractor Telescope, the 700 tells how long the focal length is (usually based on the length of the scope). The 60mm represents the diameter of the lens. This information is important to know because it will help determine clarity of the image which also makes a difference as to how much an image can be economically enlarged. Both refracting and reflecting telescopes are used in astronomy.