21.3 - Compound optics

How do binoculars, telescopes, microscopes, and cameras differ from a magnifying glass? A magnifying glass uses only one convex lens. Most optical instruments, however, use more than one optical element—lens, mirror, or disperser (e.g., prism)—and are called compound optics. Read the text aloud
Galileo’s refracting telescope of 1609 used a convex objective lens (the lens at the entrance of the telescope) and a concave eyepiece lens (the lens in front of the observer’s eye). Within two years, the German mathematician and astronomer Johannes Kepler had invented a modified design that used two convex lenses. Read the text aloud How a refracting telescope forms a magnified and inverted image of a distant object
In Kepler’s telescope, the objective lens takes the light from a distant object and forms an image at the focal point of the lens. This is not a very useful image by itself—try using a magnifying lens to look at something on the far wall! Instead, this image is used as the object for the eyepiece lens, which produces a magnified image. This then is the useful feature of a telescope: to magnify the image of a distant object, whether it is a far-away mountain peak or a galaxy. Read the text aloud
The largest refracting telescope today is 1 m in diameter, which is the maximum lens size that can be supported around its edge. Refracting telescopes also suffer from chromatic aberration: Light of different colors focuses at different points because the refractive index of glass varies with wavelength (as you will learn in Chapter 22). Read the text aloud
How a reflecting telescope focuses light to create an imageIsaac Newton built the first reflecting telescope to avoid the chromatic aberration problem in refractors. Modern telescopes also use curved mirrors instead of lenses because the large mirrors can be supported easily from the back. In a reflecting telescope, the light comes into the telescope and is redirected by a concave primary mirror. It then reflects off of a convex secondary mirror and is brought to a focus through a hole in the primary mirror. Read the text aloud Show Why build bigger telescopes?
Radio telescopes are reflecting telescopesThere are many other kinds of telescopes, such as the Very Large Array of radio telescopes in Socorro, New Mexico. Each of these radio telescopes reflects radio waves—light with very long wavelengths—off a primary and a secondary mirror onto the scientific instruments. A radio telescope is a reflecting telescope! Read the text aloud Show X-ray telescopes
What is the benefit of using mirrors instead of lenses in compound optics? Show

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