Fraunhofer's  diffraction  at  a circular aperture It has more practical  value, because  most  of the optical instruments have circular  stops. The intensity distribution in case of circular aperture looks the same as that of the rectangular slit but the dimensions of the  pattern are different. The maxima and minima take the form of  concentric  circles.  The  bright  central  maximum is called `Airy's  disc'. The angular position of minima of the diffraction pattern are given by,

D sin q = J l (d13)

where  D  is  the  diameter  of  the  slit  and J is the Bessel's function of the first kind. The first few values of J are,

1.220, 2.233, 3.238, 4.241, 5.243 etc.

As the angles q ‘s are small, sin q » q . This gives,

q ₁ = 1.22 l /D

for the first minimum. The linear radius of the Airy’s disc is given by the product of q ₁ and focal length of the lens used. See also Rayleigh’s criterion.