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 cos ⁡ ⁡ ( k x − − ω ω t + φ φ 0 ) , with initial phase φ φ 0 , angular wavenumber k and angular frequency ω ω , it can be regarded as a function of both k and x (and the time-varying part as a function of both ω ω and t ). This complementary role of k and x leads to their visualization within complementary spaces (the real space and the reciprocal space). The spatial periodicity of this wave is defined by its wavelength λ λ , where k λ λ = 2 π π ; hence the corresponding wavenumber in reciprocal space will be k = 2 π π / λ λ .