Binoculars
Binocularsare devices used to see distant objects. People used to use“binoculars” by holding two telescopes next to each other. People,however, eventually made the binoculars we use today. Thereare three types of binoculars, the primitive Galilean binoculars, thecontemporary roof-prism binoculars, and the bulky Porro-prism.  |
A picture of a pair of Galilean binoculars
Obtained from www.epinions.com/ |
TheGalilean binoculars are named after the famous Italian physicistGalileo Galilei. They are often called opera glasses, and are thecheapest and least satisfactory binoculars. They can only workadequately at very low magnification (four times of less), and have avery small field of view. These binoculars works with only lenses andno prisms. It has a concave lens for its eyepiece, and a convex lensfor the objective lens.
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A diagram of a pair of Porro-prism binoculars
Obtained from Encarta Encyclopedia 2004Click on the image to view the full-size image |
Whenyou think of binoculars, you think of the Porro-prism kind. These werenamed after a very obscure Italian scientist. His name might have beenIgnatz Porro, but no one is sure. Porro-prism lenses have two prisms tomake the length of the binoculars seem longer. In this case, the prismsact as mirrors and bend the light, therefore making the binocular tubeseem longer.
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A diagram of a pair of roof prism binoculars
Obtained from www.monkoptics.co.uk |
The roof-prism binoculars look like the cheaper Galilean binoculars andare named after the shape of the prism inside it. They have fourmechanical highlights: they are lighter than their Porro-prismcounterparts, they tolerate rough treatment better than Porro-prismbinoculars since the lenses and prisms are arranged more simply, theyare less likely to fog on the inside, and they feel better whengripped, compared to Porro-prism binoculars. Thereare down sides to roof-prisms too. The images formed by roof-prismswill not be, in general, as sharp as Porro-prism binoculars. To equalthe sharpness of Porro-prisms, roof prisms must be manufactured andaligned 300 times more accurately.
Telescopes
Telescopesare the longer, more powerful version of binoculars. They are used tosee things that are very far away, like stars. Telescopes collect andfocus light using lenses or mirrors. Optical telescopes arecategorized into two groups: refractors and reflectors. Refractors aretelescopes that use lenses, reflectors are ones using curved mirrors.
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A diagram of a refracting telescope
Obtained from Encarta Encyclopedia 2004Click on the image to view the full-size image |
Refractingtelescopes have two convex lenses. The objective lens (the lens of thetelescope where light is received) concentrates the star light to apoint called the focus. This is the place where the star can be seen orphotographed.
Thereare three kinds of reflecting telescopes: the Newtonian reflector, theCassegrain reflector, and the Nasmyth reflector. The most widely usedreflector is the Newtonian reflector.
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A diagram of a Newtonian reflector telescope
Obtained from Encarta Encyclopedia 2004Click on the image to view the full-size image |
TheNewtonian reflector is named after an English physicist, Sir IsaacNewton. It has a big bowl-shaped mirror at the end of the telescopetube to reflect the light back and to converge it, and another smalldiagonal mirror to bend the light 90 degrees to the eyepiece located atthe side of the telescope.
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A diagram of a Cassegrain reflector telescope
Obtained from www.oldham-optical.co.uk |
TheCassegrain reflector is named after the French astronomer GuillaumeCassegrain. It is slightly different from the Newtonian reflector,having a small mirror placed parallel to the large mirror (instead ofbeing diagonal to the large mirror). The light comes out from the endof the tube opposite to the side where the target object is.
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A diagram of a Nasmyth reflector telescope
Obtained from www.oldham-optical.co.uk |
TheNasmyth reflector is named after the Scottish engineer James Nasmyth.It is a derivative of the Cassegrain reflector and has anothermirror placed diagonally. This mirror is placed like inside theNewtonian reflector. It also has two straight mirrors, like theCassegrain reflector.
Themore light the telescope can collect, the fainter and the more distantthe objects it can detect. Bigger lenses or mirrors will allow thetelescope to collect more light. It is difficult, however, to makelarge lenses because they can be only supported around the rim. Biglenses sag because of their weight, generating distorted images. Bigmirrors are easier to make because they can be supported over theirentire back surface. For this reason, the biggest telescopes in theworld are all reflecting telescopes.
Microscopes
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A picture of a compound microscope
Obtained from www.microscopeworld.com |
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A diagram of a compound microscope
Obtained from Encarta EncyclopediaClick on the image to view the full-size image |
Microscopes are devices that magnify the world to let us see stuff that are too smallto see. The microscope uses two lenses to magnify the object, theeyepiece lens and the objective lens. They are both convex lenses. Theobjective lens forms an upside-down and magnified real image of theobject. The real image part means light actually passes through theplace where the image is displayed. The eyepiece magnifies this image,and creates a virtual image (this image does not have light passingthrough it).
Inother words, a microscope works almost like a refracting telescope,with just a few differences: microscopes have a much smaller objectivelens since it gathers light from a small slide rather than a distantstar. Also, the microscope tube is not as long as the telescopetube. A magnifying glass is really simple. It’s just a convex lensinside a frame.
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