One day, after pulling down your window shade, you notice that sunlight is passing through a pinhole in the shade and making a small patch of light on the far wall. Having recently studied optics in your physics class, you're not too surprised to see that the patch of light seems to be a circular diffraction pattern. It appears that the central maximum is about 4 {\rm cm} across, and you estimate that the distance from the window shade to the wall is about 3 {\rm m}.Knowing that the average wavelength of sunlight is about 550 {\rm nm}, estimate the diameter of the pinhole in mm.