The wave originating at some source does not move in a straight line but expands in a series of spherical wavelets.The wavelets add up in the forward direction to create a new wavefront, they also cancel one another, or interfere destructively, in the backward direction, so that the waves continue to propagate only in the forward direction.

To interact with the micrometer simulation, you can drag the sliders to change number of sources, wavelength and speed of propagation.