In a series of classic cloud chamber experiments in the late 1920s, English physicist Patrick Blackett obtained clear evidence of how alpha particles scatter from atomic nuclei of different masses. In this false-colour version of one of his pictures, alpha particles stream upwards through the chamber. One of them (yellow) scatters a proton (red) in the hydrogen gas filling the chamber. The proton shoots off to the right, while the heavier alpha is deflected only slightly, so that the angle between the tracks is noticeably less than 90 degrees. The tracks of non- interacting alphas are green. Compare with photos A132/003 & A132/005.