Common units used for this measurement include the reciprocal metre or inverse metre (symbol: m−1), the reciprocal centimetre or inverse centimetre (symbol: cm−1).
In optics, the dioptre is a unit equivalent to reciprocal metre.
Quantities measured in reciprocal length include: In some branches of physics, a set of natural units is adopted, such that the universal constants c, the speed of light, and ħ, the reduced Planck constant, are treated as being unity (i.e. that c = ħ = 1), which leads to mass, energy, momentum, frequency and reciprocal length all having the same unit.
As a result, reciprocal length is used as a measure of energy.
Spatial frequency is a reciprocal length, which can thus be used as a measure of energy, usually of a particle.