One of the basic particle transport problems consists of a narrow beam of light normally incident on a planar surface. As the beam progresses deeper into the system, it decays due to absorption, and broadens or “flattens” due to scattering. Closed-form expressions for the amplitude and root-mean-square radius of the beam, as functions of depth, are reported here for both the transport and diffusion equations. Also, the transport and diffusion results are compared numerically.