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Lens Lantern
A lens lantern is a small, self-contained lamp structure which may sometimes be used to serve as a lighthouse. Unlike a regular Fresnel lens, the lantern requires no housing to protect it from the weather; its glass sides would refract In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ... and magnify the light in the same fashion as would the lens. Lens lanterns were popular alternatives to lighthouses in the nineteenth century; they required less care, were cheaper to erect, and could be fairly easily placed. References See also * F. Ross Holland, Jr. '' America's Lighthouses: Their Illustrated History Since 1716''. Lighthouse fixtures {{Pharology-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Lighthouse
A lighthouse is a tower, building, or other type of physical structure designed to emit light from a system of lamps and lenses and to serve as a beacon for navigational aid, for maritime pilots at sea or on inland waterways. Lighthouses mark dangerous coastlines, hazardous shoals, reefs, rocks, and safe entries to harbors; they also assist in aerial navigation. Once widely used, the number of operational lighthouses has declined due to the expense of maintenance and has become uneconomical since the advent of much cheaper, more sophisticated and effective electronic navigational systems. History Ancient lighthouses Before the development of clearly defined ports, mariners were guided by fires built on hilltops. Since elevating the fire would improve the visibility, placing the fire on a platform became a practice that led to the development of the lighthouse. In antiquity, the lighthouse functioned more as an entrance marker to ports than as a warning signal for reefs a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Fresnel Lens
A Fresnel lens ( ; ; or ) is a type of composite compact lens developed by the French physicist Augustin-Jean Fresnel (1788–1827) for use in lighthouses. It has been called "the invention that saved a million ships." The design allows the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a lens of conventional design. A Fresnel lens can be made much thinner than a comparable conventional lens, in some cases taking the form of a flat sheet. The simpler dioptric (purely refractive) form of the lens was first proposed by Count Buffon and independently reinvented by Fresnel. The ''catadioptric'' form of the lens, entirely invented by Fresnel, has outer elements that use total internal reflection as well as refraction; it can capture more oblique light from a light source and add it to the beam of a lighthouse, making the light visible from greater distances. Description The Fresnel lens redu ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Refraction
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomenon, but other waves such as sound waves and water waves also experience refraction. How much a wave is refracted is determined by the change in wave speed and the initial direction of wave propagation relative to the direction of change in speed. For light, refraction follows Snell's law, which states that, for a given pair of media, the ratio of the sines of the angle of incidence ''θ1'' and angle of refraction ''θ2'' is equal to the ratio of phase velocities (''v''1 / ''v''2) in the two media, or equivalently, to the refractive indices (''n''2 / ''n''1) of the two media. :\frac =\frac=\frac Optical prisms and lenses use refraction to redirect light, as does the human eye. The refractive index of materials varies with the wavelengt ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Their Illustrated History Since 1716
In Modern English, ''they'' is a third-person pronoun relating to a grammatical subject. Morphology In Standard Modern English, ''they'' has five distinct word forms: * ''they'': the nominative (subjective) form * ''them'': the accusative (objective, called the 'oblique'.) and a non-standard determinative form. * ''their:'' the dependent genitive (possessive) form * ''theirs'': independent genitive form * ''themselves'': prototypical reflexive form *''themself'': derivative reflexive form (nonstandard; now chiefly used instead of "himself or herself" as a reflexive epicenity for ''they'' in pronominal reference to a singular referent) History Old English had a single third-person pronoun '' hē'', which had both singular and plural forms, and ''they'' wasn't among them. In or about the start of the 13th century, ''they'' was imported from a Scandinavian source (Old Norse ''þeir'', Old Danish, Old Swedish ''þer'', ''þair''), where it was a masculine plural demonstr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
