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In the physical sciences, the wavenumber (or wave number), also known as repetency, [1] is the spatial frequency of a wave. Ordinary wavenumber is defined as the number of wave cycles divided by length; it is a physical quantity with dimension of reciprocal length , expressed in SI units of cycles per metre or reciprocal metre (m -1 ).
Position of a point in space, not necessarily a point on the wave profile or any line of propagation d, r: m [L] Wave profile displacement Along propagation direction, distance travelled (path length) by one wave from the source point r 0 to any point in space d (for longitudinal or transverse waves) L, d, r
Mathematically, the simplest wave is a sinusoidal plane wave in which at any point the field experiences simple harmonic motion at one frequency. In linear media, complicated waves can generally be decomposed as the sum of many sinusoidal plane waves having different directions of propagation and/or different frequencies .
A molecular vibration is a periodic motion of the atoms of a molecule relative to each other, such that the center of mass of the molecule remains unchanged. The typical vibrational frequencies range from less than 10 13 Hz to approximately 10 14 Hz, corresponding to wavenumbers of approximately 300 to 3000 cm −1 and wavelengths of approximately 30 to 3 μm.
The term is also used, even more specifically, to mean a "monochromatic" or sinusoidal plane wave: a travelling plane wave whose profile () is a sinusoidal function. That is, (,) = (() +) The parameter , which may be a scalar or a vector, is called the amplitude of the wave; the scalar coefficient is its "spatial frequency"; and the scalar is its "phase shift".
If a traveling wave has a fixed shape that repeats in space or in time, it is a periodic wave. [19] Such waves are sometimes regarded as having a wavelength even though they are not sinusoidal. [20] As shown in the figure, wavelength is measured between consecutive corresponding points on the waveform.
Light rays and wave fronts are dual: if one is known, the other can be deduced. More precisely, geometrical optics is a variational problem where the “action” is the travel time T {\textstyle T} along a path, T = 1 c ∫ A B n d s {\displaystyle T={\frac {1}{c}}\int _{A}^{B}n\,ds} where n {\textstyle n} is the medium's index of refraction ...
A pendulum with a period of 2.8 s and a frequency of 0.36 Hz. For cyclical phenomena such as oscillations, waves, or for examples of simple harmonic motion, the term frequency is defined as the number of cycles or repetitions per unit of time.