HOW DO BINOCULARS WORK

Binoculars have two prisms. Inside the prisms, the light to each eye is ‘folded’ or ‘bent’ by being reflected by a pair of glass prisms. This happens so that the magnifying power of the binoculars is equal to the magnifying power of a longer telescope. The light from the object you're viewing enters the objective lens. Then it enters two prisms set at right angles to each other. These prisms are arranged at right angles so that the final image can be placed right side up. Then the light exits the ocular lens, giving you a magnified image with the correct orientation. No light is lost, because of total internal reflection. These prisms can lengthen the light path between the objective lens, and the ocular lens, thereby increasing the magnification the lens can provide, without increasing the length of the binoculars.

Although, not only can binoculars magnify an image, they can also make an image seem smaller by zooming out from the image when flipped around. This uses the same mechanism as the magnifying side of binoculars.


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Figure 1: Pair of binoculars observing and magnifying the sky.



Binocular Magnification
Binocular magnification usually ranges from 6- 20 times. That is the same thing as saying the object appears six to twenty times larger in the binoculars than it would to the unaided eye. What’s interesting is magnification is usually expressed as "X". Example: Six times magnification would be written as 6 X.



Telescopes
Compound or catadioptric telescopes are telescopes that have a mix of refractor and reflector elements in their design. The first compound telescope was made by German astronomer Bernhard Schmidt in 1930. The Schmidt telescope had a primary mirror at the back of the telescope, and a glass corrector plate in the front of the telescope to remove spherical aberration. The telescope was used primarily for photography, because it had no secondary mirror or eyepieces -- instead, photographic film was placed at the prime focus of the primary mirror. Today, the Schmidt Cassegrain design, which was invented in the 1960s, is most popular type of telescope; it uses a secondary mirror that bounces light through a hole in the primary mirror to an eyepiece.

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Figure 2: Telescope and Lenses

Glossary
Objective lens: Lens that is closest to the object. It collects lots of light from a distant object and brings that light, or image, to a point or focus.
Eyepiece lens: (ocular lens) Lens that is closest to the eye. It takes the bright light from the focus of the objective lens and magnifies it to take up more space in the retina.
Reflector: A surface that reflects.
Refractor: A telescope whose principal focusing element is a lens.
Spherical Aberration: A loss of definition in the image arising from the surface geometry of a spherical mirror or lens.


Citations
Blachford, Stacey and Kimberly A. McGrath. The Gale Encyclopedia of Science: Volume 1. Detroit: Gale Group, 2001. Print
Freudenrich, Ph.D., Craig. "How Telescopes Work" 08 November 2000. HowStuffWorks.com. <http://www.howstuffworks.com/telescope.htm> 27 September 2012.
Lowe, Lindsay ed. Introducing Physics, Light and Sound. Tucson :Brown Bear Books.2010.Print.
Reid, Struan. Invention and Discovery. Tulsa OK: USBORNE, 1986. Print.

Robertson, Bill. "Q: How Do Binoculars Work?." Science & Children 49.8 (2012): 64. Science Reference Center. Web. 12 Sept. 2012.