Four-wavevector K = ( ω c , k → ) = ( ω c , ω v p n ^ ) {\displaystyle \mathbf {K} =\left({\frac {\omega }{c}},{\vec {\mathbf {k} }}\right)=\left({\frac {\omega }{c}},{\frac {\omega }{v_{p}}}\mathbf {\hat {n}} \right)}