Overview of carbon's importance on Earth

As the free element it forms allotropes from differing kinds of carbon-carbon bonds, such as in graphite and diamond. Coal is the main source of carbon in mineral form, containing up to 95% of carbon in anthracite. Recently discovered nanostructured forms called fullerenes include buckyballs such as C60, nanotubes, and nanofibers. Because of their high strength-to-weight ratio, it is hoped that many of these carbon compounds will soon be practical for use in advanced structural composite materials.

Not only can carbon also bond with itself, but it can also form chains with a wide variety of other elements, forming nearly ten million known compounds.

Carbon-containing polymers, often with oxygen and nitrogen ions included at regular intervals in the main polymer chain, form the basis of nearly all industrial commercial plastics.

Carbon occurs in all organic life and is the basis of organic chemistry. When united with oxygen, carbon forms carbon dioxide, which is the main carbon source for plant growth. When united with hydrogen, it forms various flammable compounds called hydrocarbons which are essential to industry in the form of fossil fuels, and also other important living plant components like carotenoids and terpenes. When combined with oxygen and hydrogen, carbon can form many groups of important biological compounds including sugars, celluloses, lignans, chitins, alcohols, fats, and aromatic esters. With oxygen it forms alkaloids, and with the addition of sulfur also it forms antibiotics, amino acids and proteins. With the addition of phosphorus to these other elements, it forms DNA and RNA, the chemical codes of life.

Notable characteristics of carbon

Carbon exhibits remarkable properties, some paradoxical. Different forms include the hardest naturally occurring substance (diamond) and one of the softest substances (graphite) known. Moreover, it has a great affinity for bonding with other small atoms, including other carbon atoms, and is capable of forming multiple stable covalent bonds with such atoms. Because of these properties, carbon is known to form nearly ten million different compounds, the large majority of all chemical compounds. Carbon compounds form the basis of all life on Earth and the carbon-nitrogen cycle provides some of the energy produced by the Sun and other stars. Moreover, carbon has the highest melting/sublimation point of all elements. At atmospheric pressure it has no actual melting point as its triple point is at 10 MPa (100 bar) so it sublimates above 4000 K. Thus it remains solid at higher temperatures than the highest melting point metals like tungsten or rhenium irrespective of its allotropic form. Although thermodynamically prone to oxidation, it resists oxidation more effectively than some elements (like iron and even copper) that are weaker reducing agents at room temperature.

Although it forms an incredible variety of compounds, most forms of carbon are comparatively unreactive under normal conditions. At standard temperature and pressure, it resists all but the strongest oxidizers (such as fluorine and nitric acid). It does not react with sulfuric acid, hydrochloric acid, chlorine or any alkalis. At elevated temperatures it of course reacts with oxygen in flames and with sulfur vapors; it also combines with some metals at high temperatures to form metallic carbides and reduces such metal oxides as iron oxide.

In physics, a laser is a device that emits light through a specific mechanism for which the term laser is an acronym: Light Amplification by Stimulated Emission of Radiation. This is a combined quantum-mechanical and thermodynamical process discussed in more detail below. As a light source, a laser can have various properties, depending on the purpose for which it is designed. A typical laser emits light in a narrow, low-divergence beam and with a well-defined wavelength (corresponding to a particular color if the laser is operating in the visible spectrum). This is in contrast to a light source such as the incandescent light bulb, which emits into a large solid angle and over a wide spectrum of wavelength. These properties can be summarized in the term coherence.

A laser consists of a gain medium inside an optical cavity, with a means to supply energy to the gain medium. The gain medium is a material (gas, liquid, solid or free electrons) with appropriate optical properties. In its simplest form, a cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium. Typically, one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror.

Light of a specific wavelength that passes through the gain medium is amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium.

Part of the light that is between the mirrors (i.e., is in the cavity) passes through the partially transparent mirror and appears as a beam of light. The process of supplying the energy required for the amplification is called pumping and the energy is typically supplied as an electrical current or as light at a different wavelength. In the latter case, the light source can be a flash lamp or another laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.

The first working laser was demonstrated in May 1960 by Theodore Maiman at Hughes Research Laboratories. Recently, lasers have become a multi-billion dollar industry. The most widespread use of lasers is in optical storage devices such as compact disc and DVD players, in which the laser (a few millimeters in size) scans the surface of the disc. Other common applications of lasers are bar code readers and laser pointers. In industry, lasers are used for cutting steel and other metals and for inscribing patterns (such as the letters on computer keyboards). Lasers are also commonly used in various fields in science, especially spectroscopy, typically because of their well-defined wavelength or short pulse duration in the case of pulsed lasers. Lasers are also used for military and medical applications