44-414: Charleston Bar is a series of submerged shoals lying about eight miles southeast of Charleston, South Carolina , United States . This South Carolina state location article is a stub . You can help Misplaced Pages by expanding it . 32°43′48″N 79°51′04″W / 32.730°N 79.851°W / 32.730; -79.851 Shoal In oceanography , geomorphology , and geoscience ,
88-445: A beach , the term shoal can be applied to larger geological units that form off a coastline as part of the process of coastal erosion, such as spits and baymouth bars that form across the front of embayments and rias . A tombolo is a bar that forms an isthmus between an island or offshore rock and a mainland shore. In places of reentrance along a coastline (such as inlets , coves , rias, and bays), sediments carried by
132-433: A longshore current will fall out where the current dissipates, forming a spit. An area of water isolated behind a large bar is called a lagoon. Over time, lagoons may silt up, becoming salt marshes . In some cases, shoals may be precursors to beach expansion and dunes formation, providing a source of windblown sediment to augment such beach or dunes landforms. Since prehistoric times, humans have chosen some shoals as
176-475: A shoal is a natural submerged ridge , bank , or bar that consists of, or is covered by, sand or other unconsolidated material, and rises from the bed of a body of water close to the surface or above it, which poses a danger to navigation. Shoals are also known as sandbanks , sandbars , or gravelbars . Two or more shoals that are either separated by shared troughs or interconnected by past or present sedimentary and hydrographic processes are referred to as
220-400: A shoal complex . The term shoal is also used in a number of ways that can be either similar to, or quite different from, how it is used in geologic, geomorphic, and oceanographic literature. Sometimes, the term refers to either any relatively shallow place in a stream , lake , sea , or other body of water; a rocky area on the seafloor within an area mapped for navigation purposes; or,
264-517: A "blurring" effect in the resulting light, as it would no longer be travelling in just one direction. But this effect is not seen in nature. A correct explanation rests on light's nature as an electromagnetic wave . Because light is an oscillating electrical/magnetic wave, light traveling in a medium causes the electrically charged electrons of the material to also oscillate. (The material's protons also oscillate but as they are around 2000 times more massive, their movement and therefore their effect,
308-480: A beach, they slow down, their wave height increases and the distance between waves decreases. This behavior is called shoaling , and the waves are said to shoal. The waves may or may not build to the point where they break , depending on how large they were to begin with, and how steep the slope of the beach is. In particular, waves shoal as they pass over submerged sandbanks or reefs. This can be treacherous for boats and ships. Shoaling can also refract waves, so
352-410: A growth of vegetation on the bottom of a deep lake, that occurs at any depth, or is used as a verb for the process of proceeding from a greater to a lesser depth of water. Shoals are characteristically long and narrow (linear) ridges. They can develop where a stream , river , or ocean current promotes deposition of sediment and granular material , resulting in localized shallowing (shoaling) of
396-404: A more fundamental way be derived from the 2 or 3-dimensional wave equation . The boundary condition at the interface will then require the tangential component of the wave vector to be identical on the two sides of the interface. Since the magnitude of the wave vector depend on the wave speed this requires a change in direction of the wave vector. The relevant wave speed in the discussion above
440-497: A region of a different speed. The amount of ray bending is dependent on the amount of difference between sound speeds, that is, the variation in temperature, salinity, and pressure of the water. Similar acoustics effects are also found in the Earth's atmosphere . The phenomenon of refraction of sound in the atmosphere has been known for centuries. Beginning in the early 1970s, widespread analysis of this effect came into vogue through
484-426: A shoreline tend to strike the shore close to a perpendicular angle. As the waves travel from deep water into shallower water near the shore, they are refracted from their original direction of travel to an angle more normal to the shoreline. In underwater acoustics , refraction is the bending or curving of a sound ray that results when the ray passes through a sound speed gradient from a region of one sound speed to
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#1732780819185528-551: A site of habitation. In some early cases, the locations provided easy access to exploit marine resources. In modern times, these sites are sometimes chosen for the water amenity or view, but many such locations are prone to storm damage. An area in Northwest Alabama is commonly referred to as “ The Shoals ” by local inhabitants, and one of the cities, Muscle Shoals , is named for such landform and its abundance of Mussels . Refraction In physics , refraction
572-410: A slower rate. The light has effectively been slowed. When the light leaves the material, this interaction with electrons no longer happens, and therefore the wave packet rate (and therefore its speed) return to normal. Consider a wave going from one material to another where its speed is slower as in the figure. If it reaches the interface between the materials at an angle one side of the wave will reach
616-421: A smaller body of water from the sea, such as: The term bar can apply to landform features spanning a considerable range in size, from a length of a few meters in a small stream to marine depositions stretching for hundreds of kilometers along a coastline, often called barrier islands . They are typically composed of sand , although they could be of any granular matter that the moving water has access to and
660-414: A straight object, such as a pencil in the figure here, which is placed at a slant, partially in the water, the object appears to bend at the water's surface. This is due to the bending of light rays as they move from the water to the air. Once the rays reach the eye, the eye traces them back as straight lines (lines of sight). The lines of sight (shown as dashed lines) intersect at a higher position than where
704-422: A sunny day when using high magnification telephoto lenses and is often limiting the image quality in these cases. In a similar way, atmospheric turbulence gives rapidly varying distortions in the images of astronomical telescopes limiting the resolution of terrestrial telescopes not using adaptive optics or other techniques for overcoming these atmospheric distortions . Air temperature variations close to
748-439: Is a navigation or grounding hazard, with a depth of water of 6 fathoms (11 meters) or less. It therefore applies to a silt accumulation that shallows the entrance to or course of a river, or creek. A bar can form a dangerous obstacle to shipping, preventing access to the river or harbor in poor weather conditions or at some states of the tide . In addition to longshore bars discussed above that are relatively small features of
792-553: Is a clinical test in which a phoropter may be used by the appropriate eye care professional to determine the eye's refractive error and the best corrective lenses to be prescribed. A series of test lenses in graded optical powers or focal lengths are presented to determine which provides the sharpest, clearest vision. Refractive surgery is a medical procedure to treat common vision disorders. Water waves travel slower in shallower water. This can be used to demonstrate refraction in ripple tanks and also explains why waves on
836-436: Is a sedimentary deposit formed at a harbor entrance or river mouth by the deposition of freshwater sediment or by the action of waves on the sea floor or on up-current beaches. Where beaches are suitably mobile, or the river's suspended or bed loads are large enough, deposition can build up a sandbar that completely blocks a river mouth and dams the river. It can be a seasonally natural process of aquatic ecology , causing
880-409: Is also responsible for rainbows and for the splitting of white light into a rainbow-spectrum as it passes through a glass prism . Glass and water have higher refractive indexes than air. When a beam of white light passes from air into a material having an index of refraction that varies with frequency (and wavelength), a phenomenon known as dispersion occurs, in which different coloured components of
924-427: Is capable of shifting around (for example, soil , silt , gravel , cobble , shingle , or even boulders ). The grain size of the material comprising a bar is related to the size of the waves or the strength of the currents moving the material, but the availability of material to be worked by waves and currents is also important. Wave shoaling is the process when surface waves move towards shallow water, such as
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#1732780819185968-418: Is far smaller). A moving electrical charge emits electromagnetic waves of its own. The electromagnetic waves emitted by the oscillating electrons interact with the electromagnetic waves that make up the original light, similar to water waves on a pond, a process known as constructive interference . When two waves interfere in this way, the resulting "combined" wave may have wave packets that pass an observer at
1012-418: Is the phase velocity of the wave. This is typically close to the group velocity which can be seen as the truer speed of a wave, but when they differ it is important to use the phase velocity in all calculations relating to refraction. A wave traveling perpendicular to a boundary, i.e. having its wavefronts parallel to the boundary, will not change direction even if the speed of the wave changes. Refraction
1056-441: 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
1100-414: The human eye . The refractive index of materials varies with the wavelength of light, and thus the angle of the refraction also varies correspondingly. This is called dispersion and causes prisms and rainbows to divide white light into its constituent spectral colors . A correct explanation of refraction involves two separate parts, both a result of the wave nature of light. As described above,
1144-519: The speed of light is slower in a medium other than vacuum. This slowing applies to any medium such as air, water, or glass, and is responsible for phenomena such as refraction. When light leaves the medium and returns to a vacuum, and ignoring any effects of gravity , its speed returns to the usual speed of light in vacuum, c . Common explanations for this slowing, based upon the idea of light scattering from, or being absorbed and re-emitted by atoms, are both incorrect. Explanations like these would cause
1188-400: The actual rays originated. This causes the pencil to appear higher and the water to appear shallower than it really is. The depth that the water appears to be when viewed from above is known as the apparent depth . This is an important consideration for spearfishing from the surface because it will make the target fish appear to be in a different place, and the fisher must aim lower to catch
1232-415: The air density and thus vary with air temperature and pressure . Since the pressure is lower at higher altitudes, the refractive index is also lower, causing light rays to refract towards the earth surface when traveling long distances through the atmosphere. This shifts the apparent positions of stars slightly when they are close to the horizon and makes the sun visible before it geometrically rises above
1276-405: The apparent depth approaches zero, albeit reflection increases, which limits observation at high angles of incidence. Conversely, the apparent height approaches infinity as the angle of incidence (from below) increases, but even earlier, as the angle of total internal reflection is approached, albeit the image also fades from view as this limit is approached. The refractive index of air depends on
1320-474: The beach slopes more gradually at one end than the other. Sandbars, also known as a trough bars, form where the waves are breaking, because the breaking waves set up a shoreward current with a compensating counter-current along the bottom. Sometimes this occurs seaward of a trough (marine landform). Sand carried by the offshore moving bottom current is deposited where the current reaches the wave break. Other longshore bars may lie further offshore, representing
1364-406: The break point of even larger waves, or the break point at low tide. In Russian tradition of geomorphology , a peresyp is a sandbar that rises above the water level (like a spit ) and separates a liman or a lagoon from the sea. Unlike tombolo bars, a peresyp seldom forms a contiguous strip and usually has one or several channels that connect the liman and the sea. A harbor or river bar
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1408-519: 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 θ 1 {\displaystyle {\theta _{1}}} and angle of refraction θ 2 {\displaystyle {\theta _{2}}} is equal to the ratio of phase velocities v 1 v 2 {\textstyle {\frac {v_{1}}{v_{2}}}} in
1452-433: The fish. Conversely, an object above the water has a higher apparent height when viewed from below the water. The opposite correction must be made by an archer fish . For small angles of incidence (measured from the normal, when sin θ is approximately the same as tan θ ), the ratio of apparent to real depth is the ratio of the refractive indexes of air to that of water. But, as the angle of incidence approaches 90°,
1496-422: The formation of estuaries and wetlands in the lower course of the river. This situation will persist until the bar is eroded by the sea, or the dammed river develops sufficient head to break through the bar. The formation of harbor bars that prevent access for boats and shipping can be the result of: In a nautical sense, a bar is a shoal, similar to a reef : a shallow formation of (usually) sand that
1540-441: The horizon during a sunrise. Temperature variations in the air can also cause refraction of light. This can be seen as a heat haze when hot and cold air is mixed e.g. over a fire, in engine exhaust, or when opening a window on a cold day. This makes objects viewed through the mixed air appear to shimmer or move around randomly as the hot and cold air moves. This effect is also visible from normal variations in air temperature during
1584-427: The law of refraction is typically written as n 1 sin θ 1 = n 2 sin θ 2 . {\displaystyle n_{1}\sin \theta _{1}=n_{2}\sin \theta _{2}\,.} Refraction occurs when light goes through a water surface since water has a refractive index of 1.33 and air has a refractive index of about 1. Looking at
1628-417: The same thing is to consider the change in wavelength at the interface. When the wave goes from one material to another where the wave has a different speed v , the frequency f of the wave will stay the same, but the distance between wavefronts or wavelength λ = v / f will change. If the speed is decreased, such as in the figure to the right, the wavelength will also decrease. With an angle between
1672-441: The second material first, and therefore slow down earlier. With one side of the wave going slower the whole wave will pivot towards that side. This is why a wave will bend away from the surface or toward the normal when going into a slower material. In the opposite case of a wave reaching a material where the speed is higher, one side of the wave will speed up and the wave will pivot away from that side. Another way of understanding
1716-418: The surface can give rise to other optical phenomena, such as mirages and Fata Morgana . Most commonly, air heated by a hot road on a sunny day deflects light approaching at a shallow angle towards a viewer. This makes the road appear reflecting, giving an illusion of water covering the road. In medicine , particularly optometry , ophthalmology and orthoptics , refraction (also known as refractometry )
1760-593: The two media, or equivalently, to the refractive indices n 2 n 1 {\textstyle {\frac {n_{2}}{n_{1}}}} of the two media: sin θ 1 sin θ 2 = v 1 v 2 = n 2 n 1 {\displaystyle {\frac {\sin \theta _{1}}{\sin \theta _{2}}}={\frac {v_{1}}{v_{2}}}={\frac {n_{2}}{n_{1}}}} Optical prisms and lenses use refraction to redirect light, as does
1804-404: The water. Marine shoals also develop either by the in-place drowning of barrier islands as the result of episodic sea level rise or by the erosion and submergence of inactive delta lobes . Shoals can appear as a coastal landform in the sea , where they are classified as a type of ocean bank , or as fluvial landforms in rivers, streams, and lakes . A shoal–sandbar may seasonally separate
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1848-741: The wave fronts and the interface and change in distance between the wave fronts the angle must change over the interface to keep the wave fronts intact. From these considerations the relationship between the angle of incidence θ 1 , angle of transmission θ 2 and the wave speeds v 1 and v 2 in the two materials can be derived. This is the law of refraction or Snell's law and can be written as sin θ 1 sin θ 2 = v 1 v 2 . {\displaystyle {\frac {\sin \theta _{1}}{\sin \theta _{2}}}={\frac {v_{1}}{v_{2}}}\,.} The phenomenon of refraction can in
1892-402: The waves change direction. For example, if waves pass over a sloping bank which is shallower at one end than the other, then the shoaling effect will result in the waves slowing more at the shallow end. Thus, the wave fronts will refract, changing direction like light passing through a prism. Refraction also occurs as waves move towards a beach if the waves come in at an angle to the beach, or if
1936-529: The white light are refracted at different angles, i.e., they bend by different amounts at the interface, so that they become separated. The different colors correspond to different frequencies and different wavelengths. For light, the refractive index n of a material is more often used than the wave phase speed v in the material. They are directly related through the speed of light in vacuum c as n = c v . {\displaystyle n={\frac {c}{v}}\,.} In optics , therefore,
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