Reciprocal space comes into play regarding waves, both classical and quantum mechanical. Because a sinusoidal plane wave with unit amplitude can be written as an oscillatory term formula_1, with initial phase formula_2, angular wavenumber formula_3 and angular frequency formula_4, it can be regarded as a function of both formula_3 and formula_6 (and the time-varying part as a function of both formula_4 and formula_8). This complementary role of formula_3 and formula_6 leads to their visualisation within complementary spaces. The spatial periodicity of this wave is defined by its wavelength formula_11, where formula_12; hence the corresponding wavenumber in reciprocal space will be formula_13.