Diffraction of a circular aperture

[groosagi]

Homework Statement

A collimated laser beam ( λ=600 nm) having an intensity I=10 W/m2 is incident
perpendicularly on an opaque screen containing a circular hole with diameter
D=3.6 mm. Calculate the intensity at a point on the central axis that is distanced
180 cm from the screen.

Homework Equations

I=4I₀$$\left($$$$\frac{J(kaq/R)}{(ka/R)}$$)^2

The Attempt at a Solution

firstly I am confused if i should treat this as Fraunhofer or Fresnel diffraction. I know that for a circular aperture it will form an airy disk, but that will leave the bessel function = 0, which means the resulting intensity is zero. (that would make sense if the center is dark, ut it should be bright.

I really think I am missing something simple here. please help.

 

[ehild]

It is true that the J1(x) Bessel function is zero at x=0, but J1(x)/x has a finite limit.

ehild