The far-field diffraction of a plane wave incident on a circular aperture is often referred to as the Airy disk. The variation in intensity with angle is given by I ( θ θ ) = I 0 ( 2 J 1 ( k a sin ⁡ ⁡ θ θ ) k a sin ⁡ ⁡ θ θ ) 2 , where a is the radius of the circular aperture, k is equal to 2 π π / λ λ and J 1 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.