Problem 6.58

A parallel stream of monoenergetic electrons falls normally on a diaphragm with narrow square slit of width $b=1.0 \mu \mathrm{m}$. Find the velocity of the electrons if the width of the central diffraction maximum formed on a screen located at a distance $l=50 \mathrm{~cm}$ from the slit is equal to $\Delta x=0.36 \mathrm{~mm}$.