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.