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 nucleus (red) in the nitrogen gas filling the chamber. The alpha rebounds from the much heavier nitrogen nucleus, which travels upwards a short distance. The angle between the tracks is considerably more than 90 degrees. The tracks of non-interacting alphas are green. Compare with photos A132/003 & A132/004.