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Typical pair of single vision glasses. Single vision lenses correct for only one distance. If they correct for far distance, the person must accommodate to see up close. If the person cannot accommodate, they may need a separate correction for near distances, or else use a multifocal lens (see below).
Adjustable-focus eyeglasses might be used to replace bifocals or trifocals, or might be used to produce cheaper single-vision glasses (since they do not have to be custom-manufactured for every person). Pinhole glasses are a type of corrective glasses that do not use a lens. Pinhole glasses do not actually refract the light or change focal length.
Prism glass can be found on sidewalks, where it is known as vault lighting, [15] in windows, partitions, and canopies, where it is known as prism tiles, and as deck prisms, which were used to light spaces below deck on sailing ships. It could be highly ornamented; Frank Lloyd Wright created over forty different designs for prism tiles. [16]
An extended hemispherical lens is a special type of plano-convex lens, in which the lens's curved surface is a full hemisphere and the lens is much thicker than the radius of curvature. Another extreme case of a thick convex lens is a ball lens, whose shape is completely round. When used in novelty photography it is often called a "lensball".
Recreation is any human activity done for pleasure (or "play") during leisure time as opposed to those activities that are necessary for survival.Historically, this means that recreation only existed after human society reached the stage where leisure time existed, perhaps during the late stone age (Upper Paleolithic), as shown by the first appearance of cave paintings and musical instruments.
Before the introduction of image intensifiers, night glasses were the only method of night vision, and thus were widely utilized, especially at sea. Second World War era night glasses usually had a lens diameter of 56 mm or more with magnification of seven or eight. Major drawbacks of night glasses are their large size and weight.
The wedge prism is a prism with a shallow angle between its input and output surfaces. This angle is usually 3 degrees or less. Refraction at the surfaces causes the prism to deflect light by a fixed angle. When viewing a scene through such a prism, objects will appear to be offset by an amount that varies with their distance from the prism.
The borosilicate glass Schott BK7 (glass code 517642) is an extremely common crown glass, used in precision lenses. Borosilicates contain about 10% boric oxide, have good optical and mechanical characteristics, and are resistant to chemical and environmental damage.
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