In optical engineering, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be combinations of several optical elements, they are used in microscopes, cameras, slide projectors, CD players and many other optical instruments. Objectives are called object lenses, object glasses, or objective glasses; the objective lens of a microscope is the one at the bottom near the sample. At its simplest, it is a high-powered magnifying glass, with short focal length; this is brought close to the specimen being examined so that the light from the specimen comes to a focus inside the microscope tube. The objective itself is a cylinder containing one or more lenses that are made of glass. Microscope objectives are characterized by two parameters: numerical aperture; the magnification ranges from 4× to 100×. It is combined with the magnification of the eyepiece to determine the overall magnification of the microscope.
Numerical aperture for microscope lenses ranges from 0.10 to 1.25, corresponding to focal lengths of about 40 mm to 2 mm, respectively. A typical microscope has three or four objective lenses with different magnifications, screwed into a circular "nosepiece" which may be rotated to select the required lens; these lenses are color coded for easier use. The least powerful lens is called the scanning objective lens, is a 4× objective; the second lens is referred to as the small objective lens and is a 10× lens. The most powerful lens out of the three is referred to as the large objective lens and is 40–100×; some microscopes use an oil-immersion or water-immersion lens, which can have magnification greater than 100, numerical aperture greater than 1. These objectives are specially designed for use with refractive index matching oil or water, which must fill the gap between the front element and the object; these lenses give greater resolution at high magnification. Numerical apertures as high as 1.6 can be achieved with oil immersion.
Camera lenses need to cover a large focal plane so are made up of a number of optical lens elements to correct optical aberrations. Image projectors use objective lenses that reverse the function of a camera lens, with lenses designed to cover a large image plane and project it at a distance onto another surface. In a telescope the objective is the lens at the front end of a refractor or the image-forming primary mirror of a reflecting or catadioptric telescope. A telescope's light-gathering power and angular resolution are both directly related to the diameter of its objective lens or mirror; the larger the objective, the dimmer the object it can view and the more detail it can resolve. List of telescope parts and construction
In geometrical optics, a focus called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle; this non-ideal focusing may be caused by aberrations of the imaging optics. In the absence of significant aberrations, the smallest possible blur circle is the Airy disc, caused by diffraction from the optical system's aperture. Aberrations tend to get worse as the aperture diameter increases, while the Airy circle is smallest for large apertures. An image, or image point or region, is in focus if light from object points is converged as much as possible in the image, out of focus if light is not well converged; the border between these is sometimes defined using a "circle of confusion" criterion. A principal focus or focal point is a special focus: For a lens, or a spherical or parabolic mirror, it is a point onto which collimated light parallel to the axis is focused.
Since light can pass through a lens in either direction, a lens has two focal points – one on each side. The distance in air from the lens or mirror's principal plane to the focus is called the focal length. Elliptical mirrors have two focal points: light that passes through one of these before striking the mirror is reflected such that it passes through the other; the focus of a hyperbolic mirror is either of two points which have the property that light from one is reflected as if it came from the other. Diverging lenses and convex mirrors do not focus a collimated beam to a point. Instead, the focus is the point from which the light appears to be emanating, after it travels through the lens or reflects from the mirror. A convex parabolic mirror will reflect a beam of collimated light to make it appear as if it were radiating from the focal point, or conversely, reflect rays directed toward the focus as a collimated beam. A convex elliptical mirror will reflect light directed towards one focus as if it were radiating from the other focus, both of which are behind the mirror.
A convex hyperbolic mirror will reflect rays emanating from the focal point in front of the mirror as if they were emanating from the focal point behind the mirror. Conversely, it can focus rays directed at the focal point, behind the mirror towards the focal point, in front of the mirror as in a Cassegrain telescope. Autofocus Cardinal point Defocus aberration Depth of field Depth of focus Far point Focus Fixed focus Bokeh Focus stacking Focal Plane Manual focus
History of the telescope
The earliest known telescope appeared in 1608 in the Netherlands when an eyeglass maker named Hans Lippershey tried to obtain a patent on one. Although Lippershey did not receive his patent, news of the new invention soon spread across Europe; the design of these early refracting telescopes consisted of a convex objective lens and a concave eyepiece. Galileo applied it to astronomy. In 1611, Johannes Kepler described how a far more useful telescope could be made with a convex objective lens and a convex eyepiece lens and by 1655 astronomers such as Christiaan Huygens were building powerful but unwieldy Keplerian telescopes with compound eyepieces. Isaac Newton is credited with building the first reflector in 1668 with a design that incorporated a small flat diagonal mirror to reflect the light to an eyepiece mounted on the side of the telescope. Laurent Cassegrain in 1672 described the design of a reflector with a small convex secondary mirror to reflect light through a central hole in the main mirror.
The achromatic lens, which reduced color aberrations in objective lenses and allowed for shorter and more functional telescopes, first appeared in a 1733 telescope made by Chester Moore Hall, who did not publicize it. John Dollond learned of Hall's invention and began producing telescopes using it in commercial quantities, starting in 1758. Important developments in reflecting telescopes were John Hadley's production of larger paraboloidal mirrors in 1721; the Ritchey-Chretien variant of Cassegrain reflector was invented around 1910, but not adopted until after 1950. During the period 1850–1900, reflectors suffered from problems with speculum metal mirrors, a considerable number of "Great Refractors" were built from 60 cm to 1 metre aperture, culminating in the Yerkes Observatory refractor in 1897. A number of 4-metre class telescopes were built on superior higher altitude sites including Hawaii and the Chilean desert in the 1975–1985 era; the development of the computer-controlled alt-azimuth mount in the 1970s and active optics in the 1980s enabled a new generation of larger telescopes, starting with the 10-metre Keck telescopes in 1993/1996, a number of 8-metre telescopes including the ESO Very Large Telescope, Gemini Observatory and Subaru Telescope.
The era of radio telescopes was born with Karl Guthe Jansky's serendipitous discovery of an astronomical radio source in 1931. Many types of telescopes were developed in the 20th century for a wide range of wavelengths from radio to gamma-rays; the development of space observatories after 1960 allowed access to several bands impossible to observe from the ground, including X-rays and longer wavelength infrared bands. Objects resembling lenses date back 4000 years although it is unknown if they were used for their optical properties or just as decoration. Greek accounts of the optical properties of water filled spheres followed by many centuries of writings on optics, including Ptolemy in his Optics, who wrote about the properties of light including reflection and color, followed by Ibn Sahl and Ibn Al-Haytham. Actual use of lenses dates back to the widespread manufacture and use of eyeglasses in Northern Italy beginning in the late 13th century; the invention of the use of concave lenses to correct near-sightedness is ascribed to Nicholas of Cusa in 1451.
The first record of a telescope comes from the Netherlands in 1608. It is in a patent filed by Middelburg spectacle-maker Hans Lippershey with the States General of the Netherlands on 2 October 1608 for his instrument "for seeing things far away as if they were nearby". A few weeks another Dutch instrument-maker, Jacob Metius applied for a patent; the States General did not award a patent since the knowledge of the device seemed to be ubiquitous but the Dutch government awarded Lippershey with a contract for copies of his design. The original Dutch telescopes were composed of a convex and a concave lens—telescopes that are constructed this way do not invert the image. Lippershey's original design had only 3x magnification. Telescopes seem to have been made in the Netherlands in considerable numbers soon after this date of "invention", found their way all over Europe. In 1655 Dutch diplomat William de Boreel tried to solve the mystery of, he had a local magistrate in Middelburg follow up on Boreel's childhood and early adult recollections of a spectacle maker named "Hans" who he remembered as the inventor of the telescope.
The magistrate was contacted by a unknown claimant, Middelburg spectacle maker Johannes Zachariassen, who testified that his father, Zacharias Janssen invented the telescope and the microscope as early as 1590. This testimony seemed convincing to Boreel, who now recollected that Zacharias and his father, Hans Martens, must have been who he remembered. Boreel's conclusion that Zacharias Janssen invented the telescope a little ahead of another spectacle maker, Hans Lippershey, was adopted by Pierre Borel in his 1656 book De vero telescopii inventore. Discrepancies in Boreel's investigation and Zacharias
In physics refraction is the change in direction of a wave passing from one medium to another or from a gradual change in the medium. Refraction of light is the most observed phenomenon, but other waves such as sound waves and water waves 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 in the two media, or equivalently, to the indices of refraction of the two media. Sin θ 1 sin θ 2 = v 1 v 2 = n 2 n 1 Optical prisms and lenses utilize refraction to redirect light, as does the human eye; the refractive index of materials varies with the wavelength of light, thus the angle of the refraction varies correspondingly. This is called dispersion and causes prisms and rainbows to divide white light into its constituent spectral colors.
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 the second material first, 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 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 decrease. With an angle between 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 θ1, angle of transmission θ2 and the wave speeds v1 and v2 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; the phenomenon of refraction can in a more fundamental way be derived from the 2 or 3-dimensional wave equation. The boundary condition at the interface will 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 is the phase velocity of the wave. This is 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 if the speed of the wave changes.
Refraction of light can be seen in many places in our everyday life. It makes objects under a water surface appear closer than they are, it is what optical lenses are based on, allowing for instruments such as glasses, binoculars and the human eye. Refraction is responsible for some natural optical phenomena including rainbows and mirages. For light, the refractive index n of a material is more 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. In optics, the law of refraction is written as n 1 sin θ 1 = n 2 sin θ 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 a straight object, such as a pencil in the figure here, placed at a slant in the water, the object appears to bend at the water's surface; this is due to the bending of light rays. Once the rays reach the eye, the eye traces them back as straight lines.
The lines of sight intersect at a higher position than. This causes the pencil to appear higher and the water to appear shallower than it 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, the fisher must aim lower to catch the fish. Conversely
Leonardo Donà, or Donato was the 90th Doge of Venice, reigning from January 10, 1606 until his death. His reign is chiefly remembered for Venice's dispute with the papacy, which resulted in Pope Paul V placing a papal interdict on Venice 1606–1607; the son of Giovanni Battista Donato and Giovanna Corner, Donato was born into a merchant family. Through his shrewd business sense, he was able to turn his family's average amount of wealth into a fortune, his wealth established, Donato began a public career in Venice, serving in turn as the Venetian ambassador to Constantinople, podestà of Venice, as governor and Procurator of St Mark's. Donato served as the Venetian ambassador to the Vatican and lived at Rome for many years, his opposition to the ambitions of the papacy led him to conflict with Cardinal Borghese, the future Pope Paul V. Donato's staunchly anti-papal stance led to rumours that he was secretly a Protestant, although historians have not found any evidence of this. Donato became one of the candidates for Doge upon the death of Marino Grimani on December 25, 1605.
Donato faced two opponents in this election, but received both of their support, resulting in his election as Doge on January 10, 1606. Donato inherited a conflict with the papacy from Grimani: Between 1601 and 1604, under Grimani's leadership, had passed a number of laws limiting the power of the papacy within the Republic of Venice and withdrawing a number of clerical privileges; this came to a head in late 1605 when Venice charged two priests as common criminals, thus denying their clerical immunity from facing charges in secular courts. On December 10, 1605, two weeks before Grimani's death, Pope Paul V sent a formal protest to Venice. Shortly after his election as Doge, Donato, at the urging of Paolo Sarpi, rejected Paul V's protest; as a result, in April 1606, Paul V issued a papal interdict on Venice, thus excommunicating the entire Venetian population. At Sarpi's urging, Donato ordered all Roman Catholic clergy to ignore the Pope's interdict and continue to perform the mass, on pain of immediate expulsion from the Venetian Republic.
The Venetian clergy all continued to perform mass, except for the Jesuits, who left the Republic rather than violate the papal interdict. The Jesuits would not return to Venice until 1655. Donato and Sarpi were personally excommunicated by Paul V; the Kingdom of France acted as a mediator in the dispute between the papacy. On April 21, 1607, a deal was reached under which the two priests that Venice had charged as common criminals would be handed over to French custody, and, in exchange, the pope would remove the interdict against Venice; the remainder of Donato's reign as Doge is without note. Donato was not at all popular with the Venetian crowd, so, after his first year as Doge, Donato restricted his public appearances as Doge. Many rumours circulated about the reclusive Donato during these years, but none were substantiated, he died on July 16, 1612. This article is based on this article from Italian Wikipedia
A telescope is an optical instrument that makes distant objects appear magnified by using an arrangement of lenses or curved mirrors and lenses, or various devices used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. The first known practical telescopes were refracting telescopes invented in the Netherlands at the beginning of the 17th century, by using glass lenses, they were used for both terrestrial applications and astronomy. The reflecting telescope, which uses mirrors to collect and focus light, was invented within a few decades of the first refracting telescope. In the 20th century, many new types of telescopes were invented, including radio telescopes in the 1930s and infrared telescopes in the 1960s; the word telescope now refers to a wide range of instruments capable of detecting different regions of the electromagnetic spectrum, in some cases other types of detectors. The word telescope was coined in 1611 by the Greek mathematician Giovanni Demisiani for one of Galileo Galilei's instruments presented at a banquet at the Accademia dei Lincei.
In the Starry Messenger, Galileo had used the term perspicillum. The earliest existing record of a telescope was a 1608 patent submitted to the government in the Netherlands by Middelburg spectacle maker Hans Lippershey for a refracting telescope; the actual inventor is unknown but word of it spread through Europe. Galileo heard about it and, in 1609, built his own version, made his telescopic observations of celestial objects; the idea that the objective, or light-gathering element, could be a mirror instead of a lens was being investigated soon after the invention of the refracting telescope. The potential advantages of using parabolic mirrors—reduction of spherical aberration and no chromatic aberration—led to many proposed designs and several attempts to build reflecting telescopes. In 1668, Isaac Newton built the first practical reflecting telescope, of a design which now bears his name, the Newtonian reflector; the invention of the achromatic lens in 1733 corrected color aberrations present in the simple lens and enabled the construction of shorter, more functional refracting telescopes.
Reflecting telescopes, though not limited by the color problems seen in refractors, were hampered by the use of fast tarnishing speculum metal mirrors employed during the 18th and early 19th century—a problem alleviated by the introduction of silver coated glass mirrors in 1857, aluminized mirrors in 1932. The maximum physical size limit for refracting telescopes is about 1 meter, dictating that the vast majority of large optical researching telescopes built since the turn of the 20th century have been reflectors; the largest reflecting telescopes have objectives larger than 10 m, work is underway on several 30-40m designs. The 20th century saw the development of telescopes that worked in a wide range of wavelengths from radio to gamma-rays; the first purpose built radio telescope went into operation in 1937. Since a large variety of complex astronomical instruments have been developed; the name "telescope" covers a wide range of instruments. Most detect electromagnetic radiation, but there are major differences in how astronomers must go about collecting light in different frequency bands.
Telescopes may be classified by the wavelengths of light they detect: X-ray telescopes, using shorter wavelengths than ultraviolet light Ultraviolet telescopes, using shorter wavelengths than visible light Optical telescopes, using visible light Infrared telescopes, using longer wavelengths than visible light Submillimetre telescopes, using longer wavelengths than infrared light Fresnel Imager, an optical lens technology X-ray optics, optics for certain X-ray wavelengthsAs wavelengths become longer, it becomes easier to use antenna technology to interact with electromagnetic radiation. The near-infrared can be collected much like visible light, however in the far-infrared and submillimetre range, telescopes can operate more like a radio telescope. For example, the James Clerk Maxwell Telescope observes from wavelengths from 3 μm to 2000 μm, but uses a parabolic aluminum antenna. On the other hand, the Spitzer Space Telescope, observing from about 3 μm to 180 μm uses a mirror. Using reflecting optics, the Hubble Space Telescope with Wide Field Camera 3 can observe in the frequency range from about 0.2 μm to 1.7 μm.
With photons of the shorter wavelengths, with the higher frequencies, glancing-incident optics, rather than reflecting optics are used. Telescopes such as TRACE and SOHO use special mirrors to reflect Extreme ultraviolet, producing higher resolution and brighter images than are otherwise possible. A larger aperture does not just mean that more light is collected, it enables a finer angular resolution. Telescopes may be classified by location: ground telescope, space telescope, or flying telescope, they may be classified by whether they are operated by professional astronomers or amateur astronomers. A vehicle or permanent campus containing one or more telescopes or other instruments is called an observatory. An optical telescope gathers and focuses light from the visible part of the electromagnetic spectrum. Optical telescopes increase the apparent angular size of distant objects as well as their apparent brightness. In order for the image to be observed, photographed and sent to a computer, telescopes work by employing one or
Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word refers to visible light, the visible spectrum, visible to the human eye and is responsible for the sense of sight. Visible light is defined as having wavelengths in the range of 400–700 nanometres, or 4.00 × 10−7 to 7.00 × 10−7 m, between the infrared and the ultraviolet. This wavelength means a frequency range of 430–750 terahertz; the main source of light on Earth is the Sun. Sunlight provides the energy that green plants use to create sugars in the form of starches, which release energy into the living things that digest them; this process of photosynthesis provides all the energy used by living things. Another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. With the development of electric lights and power systems, electric lighting has replaced firelight; some species of animals generate their own light, a process called bioluminescence.
For example, fireflies use light to locate mates, vampire squids use it to hide themselves from prey. The primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, polarization, while its speed in a vacuum, 299,792,458 metres per second, is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation, is experimentally found to always move at this speed in a vacuum. In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays and radio waves are light. Like all types of EM radiation, visible light propagates as waves. However, the energy imparted by the waves is absorbed at single locations the way particles are absorbed; the absorbed energy of the EM waves is called a photon, represents the quanta of light. When a wave of light is transformed and absorbed as a photon, the energy of the wave collapses to a single location, this location is where the photon "arrives."
This is. This dual wave-like and particle-like nature of light is known as the wave–particle duality; the study of light, known as optics, is an important research area in modern physics. EM radiation, or EMR, is classified by wavelength into radio waves, infrared, the visible spectrum that we perceive as light, ultraviolet, X-rays, gamma rays; the behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths, lower frequencies have longer wavelengths; when EMR interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. EMR in the visible light region consists of quanta that are at the lower end of the energies that are capable of causing electronic excitation within molecules, which leads to changes in the bonding or chemistry of the molecule. At the lower end of the visible light spectrum, EMR becomes invisible to humans because its photons no longer have enough individual energy to cause a lasting molecular change in the visual molecule retinal in the human retina, which change triggers the sensation of vision.
There exist animals that are sensitive to various types of infrared, but not by means of quantum-absorption. Infrared sensing in snakes depends on a kind of natural thermal imaging, in which tiny packets of cellular water are raised in temperature by the infrared radiation. EMR in this range causes molecular vibration and heating effects, how these animals detect it. Above the range of visible light, ultraviolet light becomes invisible to humans because it is absorbed by the cornea below 360 nm and the internal lens below 400 nm. Furthermore, the rods and cones located in the retina of the human eye cannot detect the short ultraviolet wavelengths and are in fact damaged by ultraviolet. Many animals with eyes that do not require lenses are able to detect ultraviolet, by quantum photon-absorption mechanisms, in much the same chemical way that humans detect visible light. Various sources define visible light as narrowly as 420–680 nm to as broadly as 380–800 nm. Under ideal laboratory conditions, people can see infrared up to at least 1050 nm.
Plant growth is affected by the color spectrum of light, a process known as photomorphogenesis. The speed of light in a vacuum is defined to be 299,792,458 m/s; the fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at this same speed in vacuum. Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Rømer observed one of its moons, Io. Noting discrepancies in the apparent period of Io's orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit. However, its size was not known at that time. If Rømer had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s. Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849.