Such phenomena can also be considered from a wave viewpoint, by use of the reduced Planck constant ℏ ℏ . The wave number k is the absolute value of the wave vector k → → = p → → / ℏ ℏ , which is related to the wavelength λ λ = 2 π π / k . Often, momentum transfer is given in wavenumber units in reciprocal length Q = k f − − k i