Time evolution of the heat diffusion phenomenon from the point source near the interface

The characteristic of the mathematical solution describing linear heat diffusion from an instantaneous point source near the interface of two semi-infinite domains in perfect thermal contact is discussed. Because spherical symmetry is lacking and the solution cannot be scale-transformed into a time-independent shape, the only way to grasp the geometrical information of the solution is by ad-hoc computation. The solution which was first derived more than 65 years ago is re-arranged to permit physical interpretation. Through the term-by-term analysis, the time evolution of the solution is divided into three stages and this enables systematic understanding of its nature.