A parallel stream of mono energetic electrons falls normally on a diaphragm with narrow square slit of

Question:

A parallel stream of mono energetic electrons falls normally on a diaphragm with narrow square slit of width b = 1.0μm. Find the velocity of the electrons if the width of the central diffraction maximum formed on a screen located at a distance l = 50 cm from the slit is equal to ∆x = 0.36 mm.