Answer
$7.33\;\mu m$
Work Step by Step
The radius of the central disk of a diffraction pattern is given by
$R=\frac{1.22f\lambda}{d}$
Substituting the given values, we obtain
$R=\frac{1.22\times 3.50\times 10^{-2}\times515\times10^{-9}}{3\times 10^{-3}}\;m=7.33\times10^{-6}\;m=7.33\;\mu m$
Therefore, The radius of the central disk of a diffraction pattern is $7.33\;\mu m$
