ABSTRACT
In this chapter we shall treat two of the most important observables
of quantum mechanical systems: angular momentum and spin. We
shall argue that angular momentum is the generator of rotations. In
quantummechanics, angular momentum is quantized in units of the
reduced Planck constant. The quantization of angular momentum
is dealt with by finding the angular momentum eigenvalues and
eigenstates. We shall then apply the theory of angular momentum
to study the hydrogen atom. Spin is an intrinsic angular momentum
of subatomic particles and therefore corresponds to an important
intrinsic degree of freedom. We shall discuss briefly how spin was
discovered and many of its important properties. Finally, we shall
also explore here some further subtleties of quantum theory.a
When we consider quantum systems in two or three spatial
dimensions, we need to take into account also the effect of rotations.
In this case, we need to introduce a physical quantity called the
angular momentum, that is, themomentum induced by or connected with rotations. In classical physics the angular momentum of a
particle about a given origin is defined by [see Fig. 8.1]
