The length scale below which a particle’s quantum-mechanical properties become evident in relativistic quantum mechanics. For a particle of rest mass m the Compton wavelength is ћ/mc, where ћ is the rationalized Planck constant and c is the speed of light. The Compton wavelength is so named because of its occurrence in the theory of the Compton effect, where its value for the electron is 3.8616 × 10⁻¹³ m. The Compton wavelength is sometimes defined as h/mc, with h being the Planck constant, in which case the electron value is 2.4263 × 10⁻¹² m.