Change of the paraxiality of a Gaussian beam diffracted by a circular aperture

According to the definition of the degree of paraxiality for a monochromatic light beam, the change of the paraxiality of a Gaussian beam diffracted by a circular aperture has been investigated. When a Gaussian beam is diffracted by a circular aperture, its paraxiality decreases. The absolute and the relative changes for the degree of paraxiality are determined not only by the ratio of the aperture radius to the incident wavelength, but also by the ratio of the aperture radius to the Gaussian waist size. The change of the paraxiality of a diffracted Gaussian beam is graphically illustrated with numerical examples, and the influence of the circular aperture on the change of the paraxiality is also demonstrated.