How Telescopes and Binoculars Work

The closer an object is brought to our eyes, the larger it appears. There is a limit, however, to how close to our eyes we can bring an object and still see it clearly. This is about 10 inches. Placing a lens between the eye and an object allows the object to be moved closer than 10 inches. The object then appears larger as experienced by anyone who has used a so called magnifying glass. To magnify an object greatly, would require that it be very close to the eye. This may not be practical in most cases and impossible in the case of an astronomical body or a bird in a distant tree. Here is where the optical scheme used by microscopes, telescopes and binoculars can help us out. The curved surface of a glass lens can bend light rays coming from an object in a coherent way so as to form a small image or "picture" of the object at a fixed point within the tube of the optical device. Although the image "hangs" in mid-air, it nevertheless is real. It can actually be seen by placing a white card at the image (focal) point within the tube. Once an image is formed by the objective lens, a second lens (or lenses) called an eyepiece is used to enlarge it. The eyepiece allows the eye to come very close to the image while keeping it at a comfortable distance. By selecting eyepiece lenses of different surface curvature, almost any magnification can be obtained. The actual magnification is determined by the focal length of the objective lens (distance from lens to image) divided by the focal length of the eyepiece. A 200 inch focal length objective lens used with a 1 inch focal length eyepiece produces a magnification of 200X.