DAVID PARKER / SCIENCE PHOTO LIBRARY DAVID PARKER / SCIENCE PHOTO LIBRARY
Plate from 18th century encyclopedia showing diagrams of telescope optics, and light raypaths through prisms and lenses to the eye. An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings it to focus creating an image. The eyepiece is placed near the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the eyepiece. Two drawings illustrate lightpaths through reflecting telescopes, one showing the rays reflected by an angled flat mirror through 90 degrees to an eyepiece, the other reflected back through a hole in the primary mirror.
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