We calculate the forces of a single-beam optical trap, also known as an "optical tweezers," on micron-sized dielectric spheres based on the wave optics regime. These forces have previously been calculated using a geometrical ray optics approach by Ashkin. The relatively simple approach proposed by Ashkin is based on tracing a set of equal intensity rays from a uniformly sampled microscope objective lens aperture, all passing through a single point in space, the focal point of the lens. This model does not take into account the wave nature of light and is, therefore, it cannot account for the effects of diffraction Here we propose the use of the angular spectrum method, based on the fast Fourier transform algorithm, to calculate the scalar wavefield on the surface of a microsphere, from which ray amplitudes and directions can be estimated. This allows for the Ashkin’s method to calculate forces on trapped spherical particle to be amended to account for the effects of diffraction. Numerical results are presented for a laser power of 10mW and a microscope objective with a numerical aperture of 1.25, and compared with those obtained using the traditional geometrical approach.