The simplest form of a blast wave has been described and termed the Friedlander waveform. It occurs when a high explosive detonates in a free field: that is, with no surfaces nearby with which it can interact. Blast waves have properties predicted by the physics of waves. For example, they can diffract through a narrow opening and refract as they pass through materials. Like light or sound waves, when a blast wave reaches a boundary between two materials, part of it is transmitted, part of it is absorbed, and part of it is reflected. The impedances of the two materials determine how much of each occurs.