Answers for the Teacher Only

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What Do You Think?

A refracting telescope makes a large real image of the object with a long focal-length convex lens called the objective. Then the eyepiece, which the viewer looks through, magnifies this image. Both lenses are convex. The object (a star, for instance) is very far from the objective, so it makes a real image. But this image is very close to the eyepiece so the eyepiece functions as a magnifier. Improving a refracting telescope could mean enlarging the magnification, which is the ratio of the focal lengths of the objective and eyepiece.

Although enlarging the focal length of the objective does produce a larger real image, the light that enters the objective is spread out over this larger image, which is correspondingly dimmer. Consequently, telescope improvements must include gathering as much light as possible. (See Physics Talk for a brief description of a reflecting telescope.)