Optics
Optics is the science that studies light and our interaction with it. There are two branches of optics: physical optics and geometrical optics.
Physical optics studies the nature and propagation of light. While the ancient Greeks and Arabs have some notions about light, it was not until the 17th century that science began to understand what light is. In 1690 Christiaan Huygens published his book Treatise on Light, describing light as a wave motion. Fourteen years later in 1704, Isaac Newton published his Opticks, describing light as consisting of moving particles.
Though both points of view were useful in the description of phenomena involving light, Newton's views were dominant until the 1800s. Then, scientists like Thomas Young and Augustin-Jean Fresnel described the phenomena of interference and diffraction by assuming that light was composed of waves. Waves traveling independently could combine with other waves to enhance one another, or cancel one another, in complicated but predictable patterns. In 1864 James Clerk Maxwell published his famous equations that described light as radiant energy propagated by electromagnetic waves. Maxwell's equations succeeded admirably in describing almost all properties of light that scientists knew at the time.
But subtle inconsistencies in the scientific description of the nature of blackbody radiation (a blackbody is one that absorbs all energy that strikes it, with no reflection) led Max Planck to propose in 1900 that atoms can absorb and emit energy only in small chunks, called "quanta" (from "quantum," Latin for "how much"). In 1905 Albert Einstein used this idea to explain the photoelectric effect (the emission of electrons when light falls on substances, especially metals) by assuming that light itself was composed of quanta, called photons. Physicists grappled with these ideas for the next 25 years, only to conclude (among other things) that light is both a particle and a wave. This fundamental duality was also found to be true for any particle, including electrons. In effect, the answer to the question "is light a particle or a wave" depends on how you ask, that is, on what kind of experiment you perform. Until then, it is both.
These great advances in physical optics continued through the 20th century, resulting in the development of many useful devices, such as computerized tomography (CT), the laser, fiber optics, and more. On the theoretical front, the formal description of quanta became the theory of quantum mechanics, which was combined with Einstein's theory of relativity in the theory of quantum electrodynamics, invented independently in the years following World War II by the American physicists Richard Feynman and Julian Schwinger, and the Japanese physicist Sin-Itiro Tomonaga. The description of the photon, and its interaction with charged particles like electrons, was now complete, describing phenomena ranging from the properties of television signals to subtle properties of the hydrogen atom, with astonishing degrees of accuracy.
Meanwhile, other scientists were less concerned about the fundamental nature of light as what they could do with it. Geometric optics studies the manipulation and application of light. A portion of the light radiated by an object is gathered and focused, such as by a lens, and focused with other lenses, mirrors and other devices to create an image of the object. The object may be the letters on a blackboard, focused by eyeglasses and imaged on the eye's retina, or a distant star, focused by a telescope and imaged onto a photographic plate or computer screen.
Lenses of high quality had been produced for telescopes and microscopes since the 1700s. In 1840 the German mathematician Carl Friedrich Gauss published an influential book that detailed the concept of the focal length, the distance at which a lens focuses light. Later he and others would also describe concepts like astigmatism and distortion, establishing the formal procedures of lens design for the next 100 years.
Astronomers have designed larger and larger lenses to gather more and more light, enabling them to view greater and greater distances into the universe. The world's largest lenses are the Keck II lens at Mauna Kea, Hawaii, a combination of 36 segments with a diameter of 10 m (33 ft), the 6 m lens at Nizhny Arkhyz, Russia, and a 5 m lens at the Palomar telescope in California. Astronomers have added to their observing power by sending optical telescopes into space, above the distortion of the earth's atmosphere, such as the Hubble Space Telescope. They are also developing adaptive optics, ground-based viewing which provides real-time compensation of optical disturbances in the atmosphere, and in vibrating or poorly-shaped optical components. The electronic and optical hardware used in adaptive optics clears a fuzzy image by compensating for the disturbance as fast as it's happening.
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