In this thesis we discuss the propagation of light with low numerical aperture and high numerical aperture objectives, with or without scattering media. The point spread function (PSF) of a focused beam is studied for low and high aperture situations without scattering medium. Based on the result of other group's Monte Carlo Simulation in treating focus of light in scattering tissue, a simple model has been used to describe and explain the cause of the result. The beams formed by objective lenses with different numerical aperture (NA) are discussed so we can study the influence of changing NA on the focal spot size.