If the source is known, the basic position can determine the new position and shape of the wave face at any moment. If the position and shape of a waveform are known at any time, even if the source is not known, then Higgins' principle can be used to determine the position and shape of the waveform at any time. Note that Higgins' principle or theory is sometimes called wave theory. We can describe Higgins' principle as follows: "Every point on a wave-surface will be a tiny source of movement and from those points secondary or molecular waves will emanate and spread around at the same speed. If all these molecules touch the front of the wave and draw a surface, the new position of the wave face will be found. ”