Problem 5.99

A diaphragm with round aperture, whose radius \(r\) can be varied during the experiment, is placed between a point source of light and a screen. The distances from the diaphragm to the source and the screen are equal to \(a=100 \mathrm{~cm}\) and \(b=125 \mathrm{~cm} .\) Determine the wavelength of light if the intensity maximum at the centre of the diffraction pattern of the screen is observed at \(r_{1}=1.00 \mathrm{~mm}\) and the next maximum at \(r_{2}=1.29 \mathrm{~mm}\).