Optical phenomena refer to any observable events that occur due to the interaction of light and matter .
52-535: A mirage is a naturally-occurring optical phenomenon in which light rays bend via refraction to produce a displaced image of distant objects or the sky. The word comes to English via the French (se) mirer , from the Latin mirari , meaning "to look at, to wonder at". Mirages can be categorized as "inferior" (meaning lower), "superior" (meaning higher) and " Fata Morgana ", one kind of superior mirage consisting of
104-517: 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,
156-654: A degree high (roughly the angular diameter of the Sun and Moon) and are from objects between dozens of meters and a few kilometers away. Heat haze , also called heat shimmer , refers to the inferior mirage observed when viewing objects through a mass of heated air. Common instances when heat haze occurs include images of objects viewed across asphalt concrete (also known as tarmac ), roads and over masonry rooftops on hot days, above and behind fire (as in burning candles , patio heaters , and campfires ), and through exhaust gases from jet engines . When appearing on roads due to
208-533: A distant shoreline may appear to tower and look higher (and, thus, perhaps closer) than it really is. Because of the turbulence, there appear to be dancing spikes and towers. This type of mirage is also called the Fata Morgana , or hafgerðingar in the Icelandic language . A superior mirage can be right-side up or upside-down, depending on the distance of the true object and the temperature gradient. Often
260-404: 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
312-497: 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
364-472: A series of unusually elaborate, vertically stacked images, which form one rapidly-changing mirage. In contrast to a hallucination , a mirage is a real optical phenomenon that can be captured on camera, since light rays are actually refracted to form the false image at the observer's location. What the image appears to represent, however, is determined by the interpretive faculties of the human mind. For example, inferior images on land are very easily mistaken for
416-426: 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
468-410: 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
520-487: A spherical, convex "horizon". In some situations, distant objects can be elevated or lowered, stretched or shortened with no mirage involved. A Fata Morgana (the name comes from the Italian translation of Morgan le Fay , the fairy, shapeshifting half-sister of King Arthur) is a very complex superior mirage. It appears with alternations of compressed and stretched areas, erect images, and inverted images. A Fata Morgana
572-414: 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
SECTION 10
#1732765112823624-422: 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
676-399: Is refracted by the index gradient, making it appear as if the sky is reflected by the road's surface. This might appear as a pool of liquid (usually water, but possibly others, such as oil) on the road, as some types of liquid also reflect the sky. The illusion moves into the distance as the observer approaches the miraged object giving one the same effect as approaching a rainbow. Heat haze
728-553: 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
780-591: Is a naturally occurring optical phenomenon in which light rays are bent to produce distorted or multiple images of an astronomical object . Mirages can be observed for such astronomical objects as the Sun , the Moon , the planets , bright stars , and very bright comets . The most commonly observed are sunset and sunrise mirages. Optical phenomena Many optical phenomena are closely tied to quantum mechanics . Common optical phenomena are often due to
832-453: Is also a fast-changing mirage. Fata Morgana mirages are most common in polar regions , especially over large sheets of ice with a uniform low temperature, but they can be observed almost anywhere. In polar regions, a Fata Morgana may be observed on cold days; in desert areas and over oceans and lakes, a Fata Morgana may be observed on hot days. For a Fata Morgana, temperature inversion has to be strong enough that light rays' curvatures within
884-409: 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
936-418: 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
988-421: Is not related to the atmospheric phenomenon of haze . A superior mirage is one in which the mirage image appears to be located above the real object. A superior mirage occurs when the air below the line of sight is colder than the air above it. This unusual arrangement is called a temperature inversion . During daytime, the normal temperature gradient of the atmosphere is cold air above warm air. Passing through
1040-418: 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
1092-441: 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
SECTION 20
#17327651128231144-470: The Arctic Circle . The Sun appeared to rise two weeks earlier than expected; the real Sun had still been below the horizon, but its light rays followed the curvature of Earth. This effect is often called a Novaya Zemlya mirage . For every 111.12 kilometres (69.05 mi) that light rays travel parallel to Earth's surface, the Sun will appear 1° higher on the horizon. The inversion layer must have just
1196-414: 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,
1248-882: The particle or the wave nature of light. Some are quite subtle and observable only by precise measurement using scientific instruments. A famous example is the bending of starlight by the Sun during a solar eclipse, a phenomenon that serves as evidence for the curvature of space as predicted by the theory of relativity . Atmospheric optical phenomena include: Some phenomena are yet to be conclusively explained and may possibly be some form of optical phenomena. Some consider many of these "mysteries" to simply be local tourist attractions that are not worthy of thorough investigation. Ozerov, Ruslan P.; Vorobyev, Anatoli A. (2007). "Wave Optics and Quantum–Optical Phenomena". Physics for Chemists . pp. 361–422. doi : 10.1016/B978-044452830-8/50008-8 . ISBN 978-0-444-52830-8 . Refraction In physics , refraction
1300-519: 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
1352-663: The "Bismarck" , Ludovic Kennedy describes an incident that allegedly took place below the Denmark Strait during 1941, following the sinking of the Hood . The Bismarck , while pursued by the British cruisers Norfolk and Suffolk , passed out of sight into a sea mist. Within a matter of seconds, the ship re-appeared steaming toward the British ships at high speed. In alarm the cruisers separated, anticipating an imminent attack, and observers from both ships watched in astonishment as
1404-560: The German battleship fluttered, grew indistinct and faded away. Radar watch during these events indicated that the Bismarck had in fact made no change to her course. The conditions for producing a mirage can occur at night as well as during the day. Under some circumstances mirages of astronomical objects and mirages of lights from moving vehicles, aircraft, ships, buildings, etc. can be observed at night. A mirage of an astronomical object
1456-400: 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
1508-415: 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
1560-405: The air to vary, and the variation between the hot air at the surface of the road and the denser cool air above it causes a gradient in the refractive index of the air. This produces a blurred shimmering effect , which hinders the ability to resolve the image and increases when the image is magnified through a telescope or telephoto lens . Light from the sky at a shallow angle to the road
1612-405: 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
Mirage - Misplaced Pages Continue
1664-498: The colors generated by a prism are often shown in classrooms. Optical phenomena encompass a broad range of events, including those caused by atmospheric optical properties, other natural occurrences, man-made effects, and interactions involving human vision (entoptic phenomena) . Also listed here are unexplained phenomena that could have an optical explanation and " optical illusions " for which optical explanations have been excluded. There are many phenomena that result from either
1716-519: 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
1768-433: 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°,
1820-441: 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
1872-400: The hot asphalt, it is often referred to as a "highway mirage". It also occurs in deserts; in that case, it is referred to as a "desert mirage". Both tarmac and sand can become very hot when exposed to the sun, easily being more than 10 °C (18 °F) higher than the air a meter (3.3 feet) above, enough to make conditions suitable to cause the mirage. Convection causes the temperature of
1924-456: The image appears as a distorted mixture of up and down parts. Since the earth is round, if the downward bending curvature of light rays is about the same as the curvature of Earth , light rays can travel large distances, including from beyond the horizon. This was observed and documented in 1596, when a ship in search of the Northeast passage became stuck in the ice at Novaya Zemlya , above
1976-551: The inferior mirage is stable unlike the fata morgana which can change within seconds. Since warmer air rises while cooler air (being denser ) sinks, the layers will mix, causing turbulence . The image will be distorted accordingly; it may vibrate or be stretched vertically ( towering ) or compressed vertically ( stooping ). A combination of vibration and extension are also possible. If several temperature layers are present, several mirages may mix, perhaps causing double images. In any case, mirages are usually not larger than about half
2028-504: The interaction of light from the Sun or Moon with the atmosphere, clouds, water, dust, and other particulates. A well-known example is the rainbow , which is created when sunlight is reflected and refracted through water droplets . Some phenomena, such as the green ray , are so rare they are sometimes thought to be mythical. Others, such as Fata Morganas , are commonplace in favored locations. Other phenomena are simply interesting aspects of optics , or optical effects. For instance,
2080-434: The inversion are stronger than the curvature of Earth . The rays will bend and form arcs . An observer needs to be within an atmospheric duct to be able to see a Fata Morgana. Fata Morgana mirages may be observed from any altitude within Earth's atmosphere , including from mountaintops or airplanes. Distortions of image and bending of light can produce spectacular effects. In his book Pursuit: The Chase and Sinking of
2132-427: 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
Mirage - Misplaced Pages Continue
2184-399: The reflections from a small body of water. In an inferior mirage, the mirage image appears below the real object. The real object in an inferior mirage is the (blue) sky or any distant (therefore bluish) object in that same direction. The mirage causes the observer to see a bright and bluish patch on the ground. Light rays coming from a particular distant object all travel through nearly
2236-500: The right temperature gradient over the whole distance to make this possible. In the same way, ships that are so far away that they should not be visible above the geometric horizon may appear on or even above the horizon as superior mirages. This may explain some stories about flying ships or coastal cities in the sky, as described by some polar explorers. These are examples of so-called Arctic mirages, or hillingar in Icelandic. If
2288-404: The same layers of air, and all are refracted at about the same angle . Therefore, rays coming from the top of the object will arrive lower than those from the bottom. The image is usually upside-down, enhancing the illusion that the sky image seen in the distance is a specular reflection on a puddle of water or oil acting as a mirror . While the aero-dynamics are highly active, the image of
2340-417: 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
2392-441: 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
2444-418: 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 )
2496-409: The temperature inversion, the light rays are bent down, and so the image appears above the true object, hence the name superior . Superior mirages are quite common in polar regions , especially over large sheets of ice that have a uniform low temperature. Superior mirages also occur at more moderate latitudes, although in those cases they are weaker and tend to be less smooth and stable. For example,
2548-593: 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
2600-403: The vertical temperature gradient is +12.9 °C (23.2 °F) per 100 meters/330 feet (where the positive sign means the temperature increases at higher altitudes) then horizontal light rays will just follow the curvature of Earth, and the horizon will appear flat. If the gradient is less (as it almost always is) the rays are not bent enough and get lost in space, which is the normal situation of
2652-741: 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
SECTION 50
#17327651128232704-529: 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,
#822177