Problem 5.107

A plane monochromatic light wave falls normally on a round aperture. At a distance $b=9.0 \mathrm{~m}$ from it there is a screen showing a certain diffraction pattern. The aperture diameter was decreased $\eta=3.0$ times. Find the new distance $b^{\prime}$ at which the screen should be positioned to obtain the diffraction pattern similar to the previous one but diminished $\eta$ times.