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Optics
From Wikipedia, the free encyclopedia
Optics is the branch of physics which studies the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it.[1] Optics usually describes the behavior of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.[1]
Most optical phenomena can be accounted for using the classical electromagnetic description of light. Complete electromagnetic descriptions of light are, however, often difficult to apply in practice. Practical optics is usually done using simplified models. The most common of these, geometric optics, treats light as a collection of rays that travel in straight lines and bend when they pass through or reflect from surfaces. Physical optics is a more comprehensive model of light, which includes wave effects such as diffraction and interference that cannot be accounted for in geometric optics. Historically, the ray-based model of light was developed first, followed by the wave model of light. Progress in electromagnetic theory in the 19th century led to the discovery that light waves were in fact electromagnetic radiation.
Some phenomena depend on the fact that light has both wave-like and particle-like properties. Explanation of these effects requires quantum mechanics. When considering light's particle-like properties, the light is modeled as a collection of particles called "photons". Quantum optics deals with the application of quantum mechanics to optical systems.
Optical science is relevant to and studied in many related disciplines including astronomy, various engineering fields, photography, and medicine (particularly ophthalmology and optometry). Practical applications of optics are found in a variety of technologies and everyday objects, including mirrors, lenses, telescopes, microscopes, lasers, and fiber optics.
History
Optics began with the development of lenses by the ancient Egyptians and Mesopotamians. The earliest known lenses were made from polished crystal, often quartz, and have been dated as early as 700 BC for Assyrian lenses such as the Layard/Nimrud lens.[2] The ancient Romans and Greeks filled glass spheres with water to make lenses. These practical developments were followed by the development of theories of light and vision by ancient Greek and Indian philosophers, and the development of geometrical optics in the Greco-Roman world. The word optics comes from the ancient Greek word ὀπτική, meaning appearance or look.[3] Plato first articulated his emission theory, the idea that visual perception is accomplished by rays of light emitted by the eyes and commented on the parity reversal of mirrors in Timaeus.[4] Some hundred years later, Euclid wrote a treatise entitled Optics wherein he describes the mathematical rules of perspective and describes the effects of refraction qualitatively.[5] Ptolemy, in his treatise Optics, summarizes much of Euclid and goes on to describe a way to measure the angle of refraction, though he failed to notice the empirical relationship between it and the angle of incidence.[6]
Al-Kindi (c. 801–73) was one of the earliest important writers on optics in the Islamic world. In a work known in the West as De radiis stellarum, al-Kindi resurrected Plato's emission theory[7] which had an influence on later Western scholars such as Robert Grosseteste and Roger Bacon.[8] In 984, the Persian mathematician, Ibn Sahl wrote a treatise "On Burning Mirrors and Lenses", correctly describing a law of refraction mathematically equivalent to Snell's law.[9] He used his law of refraction to compute the shapes of lenses and mirrors that focus light at a single point on the axis. In the early 11th century, Alhazen (Ibn al-Haytham) wrote his Book of Optics, which extensively documented the then-current Islamic understanding of optics and revolutionized the field.[10][11][12] It included the first descriptions of optical phenomena associated with pinholes and concave lenses,[13][14] provided the first correct explanation of vision, described various experiments using an early scientific method,[15] and greatly influenced the later development of the modern telescope.[16]
In the 13th century, Roger Bacon, inspired by Ibn al-Haytham, used parts of glass spheres as magnifying glasses, and discovered that light reflects from objects rather than being released from them. In Italy, around 1284, Salvino D'Armate invented the first wearable eyeglasses.[17] The first rudimentary telescopes were developed independently in the 1570s and 1580s by Leonard Digges,[18] Taqi al-Din[19] and Giambattista della Porta.[20]
The earliest known working telescopes were refracting telescopes, a type which relies entirely on lenses for magnification. Their development in the Netherlands in 1608 was by three individuals: Hans Lippershey and Zacharias Janssen, who were spectacle makers in Middelburg, Holland, and Jacob Metius of Alkmaar. In Italy, Galileo greatly improved upon these designs the following year. In 1668, Isaac Newton constructed the first practical reflecting telescope, which bears his name, the Newtonian reflector.[21]
The first microscope was made around 1595, also in Middelburg.[22] Three different eyeglass makers have been given credit for the invention: Lippershey (who also developed the first real telescope); Janssen; and his father, Hans. The coining of the name "microscope" has been credited to Giovanni Faber, who gave that name to Galileo's compound microscope in 1625.[23]
Optical theory progressed in the mid-17th century with treatises written by philosopher René Descartes, which explained a variety of optical phenomena including reflection and refraction by assuming that light was emitted by objects which produced it.[24] This differed substantively from ancient Greek notions that light emanated from the eye. In the late 1660s and early 1670s, Newton expanded Descartes' ideas into a corpuscle theory of light, famously showing that white light, instead of being a unique color, was really a composite of different colors that can be separated into a spectrum with a prism. In 1690, Christian Huygens proposed a wave theory for light based on suggestions that had been made by Robert Hooke in 1664. Hooke himself publicly criticized Newton's theories of light and the feud between the two lasted until Hooke's death. In 1704, Newton published Opticks and, at the time, partly because of his success in other areas of physics, he was generally considered to be the victor in the debate over the nature of light.[24]
Newtonian optics and emission theory was generally accepted until the early 19th century when Thomas Young and Augustin-Jean Fresnel conducted experiments on the interference of light that firmly established light's wave-nature. Young's famous double slit experiment showed that light followed the law of superposition, something normal particles do not follow. This work led to a theory of diffraction for light and opened an entire area of study in physical optics.[25] Wave optics was successfully unified with electromagnetic theory by James Clerk Maxwell in the 1860s.[26]
The next development in optical theory came in 1899 when Max Planck correctly modeled blackbody radiation by assuming that the exchange of energy between light and matter only occurred in discrete amounts he called quanta.[27] In 1905, Albert Einstein published the theory of the photoelectric effect that firmly established the quantization of light itself.[28][29] In 1913, Niels Bohr showed that atoms could only emit discrete amounts of energy, thus explaining the discrete lines seen in emission and absorption spectra.[30] The understanding of the interaction between light and matter, which followed from these developments, not only formed the basis of quantum optics but also was crucial for the development of quantum mechanics as a whole. The ultimate culmination was the theory of quantum electrodynamics, which explains all optics and electromagnetic processes in general as being the result of the exchange of real and virtual photons.[31]