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 $r_{2}=1.29 \mathrm{~mm}$.