In these experiments, α-Particles obtained from uranium move only in one direction as a beam. The lead block absorbs radiation from other directions. The α-Particles collide with metal foil, moving in straight lines.
A fluorescent screen is placed around the metal foil. Scattering of α-Particles is done by metal foil. Due to this scattering, after coming out of the metal foil, the α-Particles collide at different places on the screen and produce glow.
Thus the number of particles that deviated while coming out of the metal foil was estimated.