circular aperture is known as the Airy disk. The Airy disk has the following intensity distribution as a function of angle ?:

where is the radius of the circular aperture, is equal to and is a Bessel function. The smaller the aperture, the larger the spot size at a given distance, and the greater the divergence of the diffracted beams. For aperture diameters close to the wavelength of the light, the Airy disk begins to act