Chapter 28 - Physical Optics: Interference and Diffraction - Problems and Conceptual Exercises - Page 1008: 82

$31.53\mu m$

We know that $\theta_{min}=\frac{1.22\lambda}{1.36D}$ $\theta_{min}=\frac{0.897(540\times 10^{-9}m)}{4.3\times 10^{-3}m}$ $\theta_{min}=11.26\times 10^{-15}rad$ Now $y=Ltan\theta_{min}$ We plug in the known values to obtain: $y=(28\times 10^{-2}m)tan(11.26\times 10^{-5}rad)$ $y=31.53\mu m$