The one-dimensional electron density perturbation is derived by using the cold f luid equation, Poisson's equation and the continuity equation, which is generated by a driving laser pulse propagating through a tenuous plasma. The upshifting of the frequency of a trailing pulse induced by density perturbation is studied by using optical metric. The results show that it is possible that the photon wi ll gain energy from the wake field when assuming photon number to be conserved, i.e., the photon will be accelerated.