Reciprocal length

Reciprocal length or inverse length is a measurement used in several branches of science and mathematics. As the reciprocal of length, common units used for this measurement include the reciprocal metre or inverse metre (m⁻¹), the reciprocal centimetre or inverse centimetre (cm⁻¹), and, in optics, the dioptre.

Quantities measured in reciprocal length include:

• absorption coefficient or attenuation coefficient, in materials science
• curvature of a line, in mathematics
• gain, in laser physics
• magnitude of vectors in reciprocal space, in crystallography
• more generally any spatial frequency e.g. in cycles per unit length
• optical power of a lens, in optics
• rotational constant of a rigid rotor, in quantum mechanics
• wavenumber, or magnitude of a wavevector, in spectroscopy
• density of a linear feature in hydrology and other fields; see kilometre per square kilometre