Quarks are, according to the modern theory of subatomic particles, one of the three basic building blocks of all matter. The others are the leptons (which include the electron and the three types of neutrinos) and the intermediate vector bosons (which mediate the forces that bind other particles together). The stable particles of which ordinary matter is mostly composed—protons and neutrons…
Quasi-stellar radio sources (quasars) are the most distant cosmic objects observed by astronomers. Although not visible to the naked eye, quasars are also among the most energetic of cosmic phenomena. Although some quasars may be physically smaller in size than our own solar system, some quasars are calculated to be brighter than hundreds of galaxies combined. Quasars and active galaxies appear to…
Since the early 1990s, the term queer has been strategically taken up to signify a wide-ranging and unmethodical resistance to normative models of sex, gender, and sexuality. Although this use of queer marks a process of resignification as new meanings and values are associated with what was once a term of homophobic abuse, there is always an important sense in which queer maintains, even in chang…
The quetzal (Pharomachrus mocinno), also known as the resplendent quetzal or magnificent quetzal, is an astonishingly beautiful bird of tropical forests. It is a member of the trogon family (Trogonidae). The quetzal has a body length of 14 in (36 cm); in addition, the male has impressive tail streamers as long as 25 in (64 cm). The mature male has a shining green body color, with a crimson belly, …
Raccoons are foxlike carnivores of North and South America that belong to the same family (Procyonidae) as the coatis, kinkajou, and the lesser panda. The most common species is the northern raccoon (Procyon lotor), which has numerous subspecies, all with the famous black mask on their faces and rings of dark color on their tails. They are found throughout the United States, in central Canada, and…
To discuss race in Asia, race must first be defined. Changes in definitions of race over the centuries in the West make this difficult. The earliest uses of race in sixteenth-century Europe usually focused on differences arising from common ancestry, descent, or origin. These were perceived in kinship and lineage relationships, physiological differences, or even religious or mythical ancestors. Th…
Of the ideas that have appeared in Europe over the centuries, race remains one of the most politically charged and difficult to define. Almost all scholars agree that race is a social construction, signifying no actual or important human difference. Yet, no one would argue that racism, or the process of viewing human groups as defined by inherited differences and acting in such as way as to reinfo…
Radar (RAdio Detection And Ranging) is an electronic detector system that measures distance or velocity by sending a signal out and receiving its return. It can pierce fog, darkness, or any atmospheric disturbance all the way to the horizon. Within its range, it can show an observer clouds, landmass, or objects such as ships, airplanes, or spacecraft. Radar can measure distance or range to a targe…
Radial keratotomy (RK) is a surgical procedure that reduces myopia (nearsightedness), or astigmatism (diminished focus) by changing the shape of the cornea—the outermost part of the eyeball. The procedure is particularly attractive to individuals who want to avoid wearing glasses or wish to be rid of the inconvenience of contact lenses. RK is a quick, relatively painless procedure that take…
The word radiation comes from the Latin for "ray of light," and is used in a general sense to cover all forms of energy that travel through space from one place to another as "rays." Radiation may be in the form of a spray of subatomic particles, like miniature bullets from a machine gun, or in the form of electromagnetic waves, which are nothing but pure energy and whi…
Radiation detectors are devices which sense and relay information about incoming radiation. Though the name brings to mind images of nuclear power plants and science fiction films, radiation detectors have found homes in such fields as medicine, geology, physics, and biology. The term radiation refers to energies or particles given off by radioactive matter. Mostly, radiation takes the form of alp…
A radical is an uncharged atom or molecule that has an unpaired, or "free," electron. Radicals are formed when a covalent bond in an atom or molecule is split apart and the remaining pieces retain one electron of the original shared pair. These reaction products, called free radicals, are highly reactive entities that can participate in a variety of reactions. In chemical notation, r…
A radical is a symbol for the indicated root of a number, for example a square root or cube root; the term is also synonymous for the root itself. The radical ? is the symbol that calls for the root operation; the number or variable under the radical sign is called the radicand. It is common parlance to speak of the radicand as being "under the radical." It is also common to simply u…
Radio is the technology and practice that enables the transmission and reception of information carried by long-wave electromagnetic radiation. Radio makes it possible to establish wireless two-way communication between individual pairs of transmitter and receiver, and it is used for one-way broadcasts to many receivers. Radio signals can carry speech, music, telemetry, or digitally-encoded entert…
In the nineteenth century, prominent scientists such as Charles Lyell, Charles Darwin, Sir William Thomson (Lord Kelvin), and Thomas Huxley, were in continual debate about the age of the earth. The discovery of the radioactive properties of uranium in 1896 by Henri Becquerel subsequently revolutionized the way scientists measured the age of artifacts and supported the theory that the earth was con…
The nucleus of each atom has a specific number of protons and neutrons and is either stable or unstable, depending on the relative number of each. The most stable atoms are those that have an equal number of protons and neutrons. Atoms that are unstable are radioactive. An atom that is radioactive can also be called a radionuclide. Of the known nuclides (approximately 2,000), only 264 are stable, …
Radioactive waste is generated during the production of electricity by nuclear power plants, by the eventual disposal of those facilities, and during the manufacturing and disposal of nuclear weapons and machines used in medical diagnosis and treatments, academic and industrial research, and certain industrial applications. Radioactive waste produces ionizing radiation, which can damage or destroy…
Radioisotopes are extensively used in nuclear medicine to allow physicians to explore bodily structures and functions in vivo (in the living body) with a minimum of invasion to the patient. Radioisotopes are also used in radiotherapy (radiation therapy) to treat some cancers and other medical conditions that require destruction of harmful cells. Radioisotopes, containing unstable combinations of p…
Radon (usually in the form of the radon-222 isotope) is a colorless and odorless radioactive gas formed from radioactive decay. Denoted by the atomic symbol, Rn. radon has an atomic number of 86 and the atomic weight of its most stable isotope is 222. It is a colorless, odorless gas that emits radioactivity. It is classified as a noble gas based on its location on the periodic table. Radon is the …
Water droplets and light form the basis of all rainbows, which are circular arcs of color with a common center. Because only water and light are required for rainbows, one will see them in rain, spray, or even fog. A raindrop acts like a prism and separates sunlight into its individual color components through refraction, as light will do when it passes from one medium to another. When the white l…
Rainforests are temperate or tropical forests, usually occurring as old-growth ecosystems. The world sustains many types of rainforests, which differ geographically in terms of their species composition and the environmental conditions in which they occur. However, the various rainforests have broad ecological similarities. A such, temperate and tropical rainforests are considered to represent bio…
The word "random" is used in mathematics much as it is in ordinary speech. A random number is one whose choice from a set of numbers is purely a matter of chance; a random walk is a sequence of steps whose direction after each step is a matter of chance; a random variable (in statistics) is one whose size depends on events which take place as a matter of chance. Random numbers and ot…
Rangeland is uncultivated land that is suitable for grazing and browsing animals. Rangeland is one of the major types of land in the world. (Other types are: forest, desert, farmland, pasture, and urban/industrial.) Rangelands are the principal source of forage for livestock, and they also provide habitat for a great variety of native plants and animals. Rangelands are also used by people for recr…
Raptors, or birds of prey, are birds having the following three distinctive characteristics: strong grasping feet equipped with sharp talons, a hooked upper beak, and keen vision. Raptors are called birds of prey because these features allow them to be predators that hunt for their food. Many raptors are, in fact, predators. Some raptors actually hunt for and consume other birds. Other members of …
The rare genotype advantage hypothesis asserts that genotypes (the set of genes (alleles) carried by an organism) that have been rare in the recent past should have particular advantages over common genotypes under certain new and challenging environmental conditions. Rare genotype advantage can be best illustrated by a host-parasite interaction. Successful parasites are those carrying genotypes t…
A rate is a comparison of the change in one quantity, such as distance, temperature, weight, or time, to the change in a second quantity of this type. The comparison is often shown as a formula, a ratio, or a fraction, dividing the change in the first quantity by the change in the second quantity. When the changes being compared occur over a measurable period of time, their ratio determines an ave…
Rational numbers are needed because there are many quantities or measures which natural numbers or integers alone will not adequately describe. Measurement of quantities, whether length, mass, or time, is the most common situation. Rational numbers are needed, for example, if a farmer produces and wants to sell part of a bushel of wheat or a workman needs part of a pound of copper. The reason that…
In the final section of the Critique of Pure Reason, Immanuel Kant distinguishes empiricism and rationalism: The School of Athens (1510–1511) by Raphael. In Raphael's fresco, Plato is pointing towards the heavens, indicating what he saw as the unchanging truths of knowledge, while Aristotle points downwards, symbolizing his belief that learning came from human experience. SCALA /…
Rationalization is a process applied most often to the denominators of fractions, such as 5/(1 + √+2). There are two reasons for this. If someone wanted to compute a rational approximation for such an expression, doing so would entail dividing by a many-place decimal, in this case 2.41421... With a calculator it would be easy to do, but if it must be done without a calculator, the process i…
Why is the sky blue? Why are sunsets red? The answer involves Rayleigh scattering. When light strikes small particles, it bounces off in a different direction in a process called scattering. Rayleigh scattering is the scattering that occurs when the particles are smaller than the wavelength of the light. Blue light has a wavelength of about 400 nanometers, and red light has a wavelength of about 7…
Rays are members of the class Chondrichthyes, the cartilaginous fish, that includes sharks, skates, and chimeras. The flattened shape of rays makes them unique among fish. Their pectoral fins are much larger than those of other fish, and are attached the length of the body, from the head to the posterior. Rays, and their relatives the skates, comprise the order Rajiformes, which includes 318 speci…
A real number is any number which can be represented by a point on a number line. The numbers 3.5, −0.003, 2/3, π, and √2 are all real numbers. The real numbers include the rational numbers, which are those which can be expressed as the ratio of two integers, and the irrational numbers, which cannot. (In the list above, all the numbers except pi and the square root of 2 are ra…
Realism was a movement in nineteenth century Western culture that claimed to represent ordinary people and their everyday reality based on accurate observation. It challenged centuries of tradition when the highest art aspired to idealized pictorial forms and heroic subjects. Supporters considered its visual veracity to be an indication of an artist's "sincerity." Realism acqu…