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Glossary: The quantum theory
- The displacement of a wave from zero. The maximum amplitude for a wave is the height of a peak or the depth of a trough, relative to the zero displacement line.
- constructive interference. Compare with destructive interference.
- When the peaks and troughs of two interfering waves match, the amplitudes add to give the resultant wave a higher amplitude.
- destructive interference. Compare with constructive interference.
- When the peaks of one wave match the troughs of another, the waves interfere destructively. The amplitudes of the interfering waves cancel to give the resultant wave a lower amplitude.
- diffraction. diffract. Compare with effusion.
- The ability of a wave to bend around the edges of obstacles or holes. The effect is most noticeable when the obstacle or hole is comparable to the size of the wavelength.
- electromagnetic radiation. electromagnetic wave.
- A wave that involves perpendicular oscillations in the electric and magnetic fields, moving at a speed of 2.99792458×108 m/s in a vacuum away from the source. gamma rays, x-rays, ultraviolet light, visible light, infrared radiation, and radio waves are all electromagnetic waves.
- frequency. () Compare with wavelength.
- The number of cycles of a wave that move past a fixed observation point per second. The SI unit of frequency is the Hertz (Hz).
- gamma rays. (-rays) gamma radiation.
- A very high energy form of electromagnetic radiation, typically with wavelengths of less than 3 pm. Gamma rays are produced by certain nuclear decay processes, and are used to sterilize food.
- Hertz. (Hz, s-1) frequency.
- The SI unit of frequency, equal to one cycle of the wave per second (s-1).
- infrared radiation. (IR) infrared.
- Electromagnetic radiation with wavelength longer than visible light but shorter than that of microwaves. Infrared radiation is produced by hot objects; absorption of infrared radiation causes chemical bonds to vibrate.
- interference. interfering. Compare with constructive interference and destructive interference.
- The amplitudes of waves moving into the same region of space add to produce a single resultant wave. The resultang wave can have higher or lower amplitude than the component waves. See constructive interference and destructive interference.
- microwave. microwave radiation.
- Electromagnetic radiation with wavelength between 3 mm and 30 cm.
- momentum. (p)
- Momentum is a property that measures the tendency of a moving object to keep moving in the same direction. Increasing the speed of an object increases its momentum, and a heavy object will have more momentum than a lighter one moving at the same speed. For a particle with mass m and velocity v, the momentum of the particle is mv.
- A point, region, or surface where the amplitude of a standing wave is zero. The probability of finding an electron at an orbital node is zero.
- phase. in phase; out of phase; wave phase.
- 1.A phase is a part of a sample of matter that is in contact with other parts but is separate from them. Properties within a phase are homogeneous (uniform). For example, oil and vinegar salad dressing contains two phases: an oil-rich liquid, and a vinegar-rich liquid. Shaking the bottle breaks the phases up into tiny droplets, but there are still two distinct phases. 2. In wave motion, phase is the fraction of a complete cycle that has passed a fixed point since the current cycle began. The phase is often expressed as an angle, since a full cycle is 360^deg; (2 ). Two waves are "in phase" if the peaks of one wave align with the peaks of the other; they are "out of phase" if the peaks of one wave align with the troughs of the other.
- photoelectric effect.
- Ejection of electrons from an atom or molecule that has absorbed a photon of sufficient energy. The photoelectric effect is the operating principle behind "electric eyes"; it is experimental evidence for particle-like behavior of electromagnetic radiation.
- An electron ejected from an atom or molecule that has absorbed a photon.
- photon. (h) quantum; quanta.
- A discrete packet of energy associated with electromagnetic radiation. Each photon carries energy E proportional to the frequency of the radiation: E = h, where h is Planck's constant.
- Planck's constant. (h)
- A proportionality constant that relates the energy carried by a photon to its frequency. Planck's constant has a value of 6.6262 × 10-34 J s.
- quantum. quanta.
- A discrete packet of energy.
- quantum mechanics. quantum theory.
- A branch of physics that describes the behavior of objects of atomic and subatomic size.
- quantum number.
- Indices that label quantized energy states. Quantum numbers are used to describe the state of a confined electron, e. g. an electron in an atom.
- ultraviolet light. ultraviolet; ultraviolet radiation; ultraviolet region; UV.
- Electromagnetic radiation with wavelength longer than that of x-rays but shorter than that of visible light. Ultraviolet light can break some chemical bonds and cause cell damage.
- uncertainty principle. Heisenberg's uncertainty principle; Heisenberg principle; indeterminancy; indeterminancy principle.
- The exact momentum and exact location of a particle cannot be specified. Werner Heisenberg stated that the product of uncertainties in location and momentum measurements can never be smaller than h/4, where h is Planck's constant.
- visible light.
- Visible light is electromagnetic radiation with a wavelength between 400 and 750 nm.
- wavefunction. ()
- A mathematical function that gives the amplitude of a wave as a function of position (and sometimes, as a function of time and/or electron spin). Wavefunctions are used in chemistry to represent the behavior of electrons bound in atoms or molecules.
- The distance between adjacent peaks (or adjacent troughs) on a wave. Varying the wavelength of light changes its color; varying the wavelength of sound changes its pitch.
- An oscillating motion that moves outward from the source of some disturbance (ripples running away from a pebble tossed in a pond). Waves transmit the energy of the disturbance away from its source.
- A very high energy form of electromagnetic radiation (though not as high energy as gamma rays). X-rays typically have wavelengths from a few picometers up to 20 nanometers. X-rays easily penetrate soft tissue, which makes them useful in medical imaging and in radiation therapy.
- x-ray diffraction pattern.
- Interference patterns created by x-rays as they pass through a solid material. Studying x-ray diffraction patterns gives detailed information on the three-dimensional structure of crystals, surfaces, and atoms.
- x-ray spectrum. x-ray spectra.
- A set of characteristic x-ray frequencies or wavelengths produced by a substance used as a target in an x-ray tube. Each element has a characteristic x-ray spectrum, and there is a strong correlation between atomic number and the frequencies of certain lines in the x-ray spectrum.
- x-ray tube.
- A cathode ray tube that focuses energetic streams of electrons on a metal target, causing the metal to emit x-rays.
- zero point energy.
- A minimum possible energy for an atom or molecule predicted by quantum mechanics. Electrons stay in motion and bonds continue to vibrate even at absolute zero because of zero point energy.