- 行业: Printing & publishing
- Number of terms: 178089
- Number of blossaries: 0
- Company Profile:
McGraw Hill Financial, Inc. is an American publicly traded corporation headquartered in Rockefeller Center in New York City. Its primary areas of business are financial, publishing, and business services.
A phenomenon exhibited when atomic nuclei in a static magnetic field absorb energy from a radio-frequency field of certain characteristic frequencies. Nuclear magnetic resonance is a powerful analytical tool for the characterization of molecular structure, quantitative analysis, and the examination of dynamic processes. It is based on quantized spectral transitions between nuclear Zeeman levels of stable isotopes, and is unrelated to radioactivity.
Industry:Science
A phenomenon in which a physical quantity depends not only on the temperature but also on the preceding thermal history. It is usual to compare the behavior of the physical quantity while heating and the behavior while cooling through the same temperature range. The <b>illustration</b> shows the thermal hysteresis which has been observed in the behavior of the dielectric constant of single crystals of barium titanate. On heating, the dielectric constant was observed to follow the path <i>ABCD</i> and so on, cooling the path <i>DCEFG</i>.
Industry:Science
A phenomenon in which the most loosely held bonding electrons of some molecules serve to bind not two but several atoms. This contrasts with localization, a characteristic of ordinary single bonds, as in the normal paraffins, for example, methane (CH<sub>4</sub>); in ethane, (C<sub>2</sub>H<sub>6</sub>); or in the molecules water (H<sub>2</sub>O) and ammonia (NH<sub>3</sub>).
Industry:Science
A phenomenon observed in the scattering of light as it interacts with a material medium in which the incident light suffers a change in frequency due to internal energy change of the molecular scatterers. Raman scattering differs in this respect from Rayleigh scattering in which the incident and scattered light have the same frequency. Shifts in frequency are determined by the type of molecules in the scattering medium, and spectral analysis of the scattered light can provide a “fingerprint” of the chemical structure of the scatterers. Both incoherent and coherent forms of the Raman effect exist. Spontaneous Raman scattering, the usual (incoherent) form, is very weak, with Raman signals 4–5 orders of magnitude smaller than Rayleigh scattering and about 14 orders of magnitude smaller than fluorescence. In stimulated Raman scattering, a coherent form, the signals may be quite large. In addition, there are nonlinear forms of Raman scattering, including hyper-Raman scattering and coherent anti-Stokes Raman scattering (CARS).
Industry:Science
A phenomenon occurring in many electrical conductors, in which the electrons responsible for conduction undergo a collective transition into an ordered state with many unique and remarkable properties. These include the vanishing of resistance to the flow of electric current, the appearance of a large diamagnetism and other unusual magnetic effects, substantial alteration of many thermal properties, and the occurrence of quantum effects otherwise observable only at the atomic and subatomic level.
Industry:Science
A phenomenon resulting from an instability of the atomic nucleus in certain atoms whereby the nucleus experiences a spontaneous but measurably delayed nuclear transition or transformation with the resulting emission of radiation. The discovery of radioactivity by Henri Becquerel in 1896 was an indirect consequence of the discovery of x-rays a few months earlier by Wilhelm Roentgen, and marked the birth of nuclear physics.
Industry:Science
A phenomenon wherein two (or more) physical quantities bear a relationship which depends on prior history. More specifically, the response <i>Y</i> takes on different values for an increasing input <i>X</i> than for a decreasing <i>X</i>.
Industry:Science
A phosphor is a material that gives off light under some type of external stimulation, which can be an electron beam, light of a different wavelength, or a voltage or electric field. Phosphors are critical to the development and improvement of display technologies.
Industry:Science
A photoelectric device for intensifying faint astronomical images. In these devices a photoemissive surface, called the photocathode, emits electrons through the photoelectric effect. In most tubes the photocathode is semitransparent and is deposited on the inside of a transparent window that is mounted on the end of an evacuated glass or ceramic cylinder. When light from a telescope or spectrograph is imaged on the photocathode, electrons are ejected into the vacuum inside the tube. Electric fields or electric and magnetic fields in combination then accelerate and direct the photoelectrons through the device. In an image intensifier tube the photoelectrons are ultimately reimaged on a phosphor-coated output window that converts them back into visible light. An external television camera or charge-coupled device (CCD) is used to record the image. Optical gain is provided by the phosphor, which may release as many as a thousand photons for each photoelectron which crashes into it. In other devices the electron image is projected onto a solid-state diode array or a grid of anodes that is built directly into the tube.
Industry:Science
A photographic lens system specially designed to give a large image of a distant object in a camera of relatively short focal length. A telephoto lens generally consists of a positive lens system and a negative lens system, separated by a considerable distance. If color correction is desired, each of the partial systems must be color-corrected. It is usually not easy to correct distortion in a teleobjective, but occasionally it has been achieved.
Industry:Science
