The free particle (zero potential) solution of the Schrodinger equation for a plane wave (from my understanding) is ψ= A e^(ikx) + B e^(-ikx). I have 2 questions: 1) This is the so-called free "particle" solution for a plane wave. Does this same solution apply to a light wave/photon also? 2) Are BOTH the A and B expressions involved in describing the wave function? That would describe a wave moving in both directions along the x-axis, wouldn't it? Or would it be a superposition of the two waves, one moving to the left and one moving to the right?