To gain a better understanding of the nature of these spherical waves, let us go back and look at the case when l = 0. In this case, there is no angular dependence, and the amplitude depends only on the radial distance i.e. Ψ(r, t) → u (r, t). In this case, the wave equation reduces to ( ∇ ∇ 2 − − 1 c 2 ∂ ∂ 2 ∂ ∂ t 2 ) Ψ Ψ ( r , t ) = 0 , or ( ∂ ∂ 2 ∂ ∂ r 2 + 2 r ∂ ∂ ∂ ∂ r − − 1 c 2 ∂ ∂ 2 ∂ ∂ t 2 ) u ( r , t ) = 0.