Problem 5.115

A plane light wave with wavelength $\lambda=0.65$ \mum falls normally on a large glass plate whose opposite side has a ledge and an opaque strip of width $a=0.30 \mathrm{~mm}$ (Fig. 5.23 ). A screen is placed at a distance $b=110 \mathrm{~cm}$ from the plate. The height $h$ of the ledge is such that the intensity of light at point 2 of the screen is the highest possible. Making use of Fig. $5.19,$ find the ratio of intensities at points 1 and 2 .