Effect of apertures on paraxiality of plane wave

According to definition of degree of paraxiality for a monochromatic light beam, the effects of a circular aperture and a rectangular aperture on the paraxiality of a plane wave have been investigated, respectively. Analytical expressions of degree of paraxiality for a plane wave diffracted by a circular aperture and a rectangular aperture have been derived. If a plane wave is diffracted by a circular aperture or a rectangular aperture, the corresponding paraxiality decreases, which means that degree of paraxiality of the diffracted plane wave is smaller than one. Decreasing quantity of degree of paraxiality is determined by the ratio of aperture dimension to incident wavelength. The numerical results show that degree of paraxiality of the diffracted plane wave quickly augments from zero a