Interaction of a laser field with a plasma wave is studied by metric optics. Analysis shows that the frequencyupshifting of the laser pulse results from the plasma density gradient. A laser beam can be thought of as a packet ofphotons moving in a plasma and thus the laser frequency upshifting is equivalent to photon acceleration. Examination ofthe three-dimensional motion equations shows that a laser beam diffraction occurs in the presence of a radial variation ofthe plasma density. It is argued that the focusing mechanism originating from the plasma wave can curb laser diffractionso that photons may be trapped in the plasma wave and accelerated continuously.