As early as the 1300s, eyeglasses made from convex, or outward-curving, lenses had been developed and were used by people with trouble focusing on nearby objects. By 1450, eyeglasses with concave, or inward-curving, lenses had been developed for people with trouble seeing objects at a distance. Thus, the primary ingredients for making a telescope -- namely, convex and concave lenses -- had been invented. But it wasn't until 1608 that the Dutch optometrist Hans Lippershey applied for a patent on an invention that combined a convex and a concave lens to get a magnifying effect so that faraway objects could be seen as if nearby.

The Italian physicist and mathematician Galileo Galilei learned of Lippershey's "spyglass" device. Despite never having seen it, Galileo worked out the mathematics of the device. He taught himself how to grind lenses, and in a very short time, his mechanical enhancements enabled him to see objects through his telescope at greater distances than Lippershey's spyglass allowed.

Early telescopes such as Galileo's were designed to use the principles of refraction, or the bending of light rays as they pass from one medium, such as air, into and out of another medium, such as glass. The convex objective lens, located at one end of the telescope's tubular body, gathered the light from a distant object. The larger the lens, the more light it could gather. As light rays passed through the objective lens, its curvature caused the rays to converge and form an image of the viewed object near the other end of the tube, at the focal point. A concave eyepiece then magnified the tiny image for the viewer to see.

Galileo's first telescope offered limited magnification and a narrow field of view. He eventually succeeded in magnifying an object about 30 times, but realized that enlarging it any further was not practical because the field of view became too small. Galileo used his instruments to make important discoveries concerning the surface of the Moon, the moons of Jupiter, the phases of Venus, and sunspots.