Back to Bohr

One of Niels Bohr's main contributions to Physics was his assertion (backed up by experiment) that angular momentum is quantized - it can only occur in integer multiples of . Erwin Schr´'odinger showed why this was true for the wave functions of the hydrogen atom, but by that time Bohr's principle had been elevated to an empirical "law" of Physics that went well beyond the realm of atoms. Schr´'odinger also showed the peculiar nature of the quantization of : first, its magnitude obeys where can only have integer values from zero to , n being the principle quantum number for which in the case of hydrogen; second, its projection onto the z axis obeys where can take on only integer values from to . Note that Bohr's original prescription for angular momentum quantization (integer multiples of ) is actually applicable to the z component of - its projection onto the z quantization axis, which is chosen arbitrarily unless there is a magnetic field applied, in which case is always chosen along the field, .