The previous set of notes discussed wave reflections generated from boundaries. The phenomenon of reflection is far more general and occurs (except under very special circumstances) whenever there is a change in the medium of propagation. Practical applications of this include RADAR, where microwaves propagating through the atmosphere encounter another material, such as the metallic body of an aircraft or even the flesh and bone of a flock of birds, whose propagation properties are different. The resulting reflection then returns to the RADAR source, where the presence of the reflection and thus the presence of something in the air is detected. Another application is in medical ultrasound applications. Here, high frequency sound waves traveling through one material, such as amniotic fluid, encounters another material which different propagation properties, such as a baby. Again, the change in propagation generates a reflection, which can be measured to study how the baby is developing.
At a general change in medium, not all of the wave is reflected. Some of it is transmitted into the new medium, where it continues to propagation. The purpose of these notes is to understand both processes, transmission and reflection.