1) In an interference filter, the wavelength of peak
transmission.
2) In a spectrograph, the wavelength corresponding to the middle of
the range covered by the grating or grism.

Fr.: longueur d'onde de Compton, longueur d'onde Compton

The quantum wavelength of a particle with a highly relativistic velocity. The Compton
wavelength is given by h/mc, where h is Planck's constant, m
is the mass of the particle, and c the light speed. For an electron, the Compton
wavelength is about 2.4 × 10^{-10} cm, intermediate between the size of an
atomic nucleus and an atom.

Deriving the → wavelength of an undulatory phenomenon from
its → frequency, and vice versa.
1) For → electromagnetic waves:
λ = c / f, where λ is the wavelength,
c is the → speed of light in
→ meters per second
and f the frequency in → hertz. It can be written as:
λ (m) = 2.998 × 10^{8} / f (Hz).
2) For → sound waves: λ = C / f,
where C is the → sound speed. For air at
temperature 0°C, λ (m) = 332 / f (Hz).

A refinement of → Fourier analysis which enables to
simplify the description of a
complicated function in terms of a small number of coefficients.
The formal history of wavelet theory began in the early 1980s when Jean Morlet, a French
geophysicist, introduced the concept of wavelet and studied wavelet transform as
a new tool for scientific signal analysis. In 1984, his
collaboration with Alex Grossmann yielded a detailed mathematical study of the
continuous wavelet transforms and their various applications.
Although similar results had already been obtained 20-50 years earlier by
several other researchers, the rediscovery of the old concepts provided a new
method for decomposing functions.