This process provides a right-side-up image, but with low magnification and a narrow field of view. A concave (inward-curving) lens can also be used as an eyepiece, by intercepting the rays before the real image is completely formed. This virtual image can be seen only by looking through the eyepiece, which usually consists of a convex lens or lens combination, in which case the image will be upside down. Instead, though, this image is viewed (by the human eye, or perhaps a camera or other instrument) through the eyepiece, which forms a “virtual” image of the real image. The image is called real because the rays really meet there, and the image could be displayed on paper or captured on film. If the object is very distant, this image forms at the objective’s “focal point,” at a distance called the “focal length” from the lens. Such a lens is thicker in the middle than at the edge and bends light rays from a distant object toward each other to form what is called a real image. At the end of the tube toward the object to be viewed is the objective, which consists of a convex (outward-curving) lens or a combination of lenses that act as a convex lens. The refracting telescope has a closed tube. The devices used to focus this radiation vary, however, depending on the wavelength or type of radiation being studied. Because of their basic similarity, most forms of radiation can be focused by reflecting them off a curved surface or by refracting, or bending, them with glass lenses. This radiation travels through space at the speed of light in the form of waves of electric and magnetic fields. Sophisticated telescopes are used to view radiation in all parts of the electromagnetic spectrum, from long-wave radiation and radio waves to infrared radiation and visible light to much shorter-wave radiation, including ultraviolet radiation and X-rays. The ability to study the distant planets and other structures in the universe with these powerful yet simple instruments has revolutionized humankind’s understanding of the natural world.Īll telescopes gather radiation from distant objects over a large area and focus it, thereby increasing the intensity of the radiation and allowing the objects to be magnified. The word telescope is derived from the Greek words tele, meaning “far off,” and skopos, “viewer.” Even a simple homemade telescope can clearly show Saturn’s rings, Jupiter’s bands and red spot, stars, nebulae, and nearby galaxies not visible to the unaided eye. Their use for viewing celestial objects is crucial to modern astronomy and has dramatically expanded knowledge of the universe. Telescopes, sometimes used in pairs called binoculars, are commonly used to observe birds, ships, or other objects here on Earth. The first and most familiar type, however, is the optical telescope, which provides a detailed, magnified image using light in the visible part of the spectrum. More generally, the word has come to include just about any device for collecting electromagnetic or other radiation from distant sources for analysis. A telescope is essentially a device for extending the sense of sight.
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