The (time-independent) Schrödinger equation in quantum mechanics is
∇2ψ+(ϵ−bV)ψ=0,
where ϵ and b are constants and V is a given function of r, θ, ϕ for each problem. In most simple cases V is a function of r only (no θ, ϕ dependence). (Physically, V is the potential energy, and the fact that it depends only on r implies that we are dealing with central forces, for example, electrostatic or gravitational forces.) Separate the Schrödinger equation in spherical coordinates for the case V=V(r), and show that the θ, ϕ solutions are spherical harmonics (see Problem 16).