Schematic diagram of apparatus used by Geiger and Marsden to observe scattering of $\alpha$ particles past 90°. "A small fraction of the $\alpha$ particles falling upon a metal foil have their directions changed to such an ex tent that they emerge again at the side of incidence." The scattered $\alpha$ particle struck a scintillating screen where the brief flash was observed through the microscope.
From H. Geiger and E. Marsden, Proceedings of Royal Society (London) 82, 495 (1909).
This is about the alpha-particle scattering experiment. The book says that some alpha-particles were scattered with the angle larger than 90 degree, which is impossible if the particles were colliding with electrons of small mass. There's the formula of the maximum scattering angle at the bottom of the second image. I don't get how the formula is derived. What does the angle have to do with the momentum?