Editing Diffraction (section)

=== Circular aperture ===
{{Main|Airy disk}}

[[Image:Airy-pattern.svg|thumb|A computer-generated image of an '''Airy disk''']]

[[Image:Fresnel to Fraunhofer transition.gif|thumb|Diffraction pattern from a circular aperture at various distances]]

The far-field diffraction of a plane wave incident on a circular aperture is often referred to as the [[Airy disk]].  The [[Airy disk#Mathematical details|variation]] in intensity with angle is given by
<math display="block">I(\theta) = I_0 \left ( \frac{2 J_1(ka \sin \theta)}{ka \sin \theta} \right )^2 ,</math>
where <math>a</math> is the radius of the circular aperture, <math>k</math> is equal to <math>2\pi/\lambda</math> and <math>J_1</math> 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.