3.2 Quantum theory

Quantum theory describes the particles that make up matter and how they interact with each other and with energy. Quantum theory explains in principle how to calculate what will happen in any experiment involving physical or biological systems, and how to understand how our world works. The name "quantum theory" comes from the fact that the theory describes the matter and energy in the universe in terms of single indivisible units called quanta (singular quantum). Quantum theory is different from classical physics. Classical physics is an approximation of the set of rules and equations in quantum theory. Classical physics accurately describes the behavior of matter and energy in the everyday universe. For example, classical physics explains the motion of a car accelerating or of a ball flying through the air. Quantum theory, on the other hand, can accurately describe the behavior of the universe on a much smaller scale, that of atoms and smaller particles. The rules of

classical physics do not explain the behavior of matter and energy on

this small scale. Quantum theory is more general than classical physics, and in principle, it could be used to predict the behavior of any physical, chemical, or biological system. However, explaining the behavior of the everyday world with quantum theory is too complicated to be practical.

Quantum theory not only specifies new rules for describing the universe but also introduces new ways of thinking about matter and energy. The tiny particles that quantum theory describes do not have defined locations, speeds, and paths like objects described by classical physics. Instead, quantum theory describes positions and other properties of particles in terms of the chances that the property will have a certain value. For example, il allows scientists to calculate how likely it is that a particle will be in a certain position at a certain time.

Quantum description of particles allows scientists to understand how particles combine to form atoms. Quantum description of atoms helps scientists understand the chemical and physical properties of molecules, atoms, and subatomic particles. Quantum theory enabled scientists to understand the conditions of the early universe, how the Sun shines, and how atoms and molecules determine the characteristics of the material that they make up. Without quantum theory, scientists could nol have developed nuclear energy or the electric circuits that provide the basis for computers.

Quantum theory describes all of the fundamental forces—except gravitation—that physicists have found in nature. The forces that quantum theory describes are the electrical, the magnetic, the weak, and the strong. Physicists often refer to these forces as interactions, because the forces control the way particles interact with each other.

3.4 Fiber optics

Fiber Optics is the branch of optics dealing with the transmission of light through fibers or thin rods of glass or some other transparent material of high rcfractive index. If light is admitted at one end of a fiber, it can travel through the fiber with very low loss, even if the fiber is curved.

The principle on which this transmission of light depends is that of total internal reflection: Light traveling inside the fiber center, or core, strikes the outside surface at an angle of incidence greater than the critical angle, so lliat all the light is reflected toward the inside of the fiber without loss. Thus light can be transmitted over long distances by being reflected inward thousands of times. In order to avoid losses through the scattering of light by impurities on the surface of the fiber, the optical fiber core is clad with a glass layer of much lower rcfractive index; the reflections occur at the interface of the glass fiber and the cladding.

The simplest application of optical fibers is the transmission of light to locations otherwise hard to reach, for example, the bore of a dentist's drill. Also, bundles of several thousand very thin fibers assembled precisely side by side and optically polished at their ends, can be used to transmit images. Each point of the image projected on one face of the bundle is reproduced at the other end of the bundle, reconstituting the image, which can be observed through a magnifier. Image transmission by optical fibers is widely used in medical instruments for viewing inside the human body and for laser surgery, in facsimile systems, in phototypesetting, in computer graphics, and in many other applications.

Optical fibers are also being used in a wide variety of sensing devices, ranging from thermometers to gyroscopes. The potential of their applications in this field is nearly unlimited, because the light sent through them is sensitive to many environmental changes, including pressure, sound waves, and strain, as well as heat and motion. The fibers can be especially useful where electrical effects could make ordinary wiring useless, less accurate, or even hazardous. Fibers have also been developed to carry high-power laser beams for cutting and drilling.

One growing application of optical fibers is in communication. Recausc the information-carrying capacity of a signal increases with frequency, the use of laser light offers many advantages. Fiber-optic laser systems are being used in communications networks. Many- long-haul fiber communications networks for both transcontinental connections and. through undersea cables, international connections arc in operation. One advantage of optical fiber systems is the long distances that can be maintained before signal repeaters arc needed to regenerate signals. These are currently separated by about 100 km (about 62 mi), compared to about 1.5 km (about I mi) for electrical systems. Newly developed optical fiber amplifiers can extend this distance even farther.

Local area networks are another growing application for fiber optics. Unlike long-haul communications, these systems connect many local subscribers to expensive centralized equipment such as computers and printers. This system expands the utilization of equipment and can easily accommodate new users on a network. Development of new electro-optic and integrated-optic components will further expand the capability of fiber systems.