In mathematics, phase generally appears in trigonometric functions, indicating the state of the function at a certain moment. In physics, physical models such as simple harmonic vibration and alternating circuit are mostly described by sine and cosine functions. In different models, the phase can represent the state of the system at that time. In fact, it is always a time to turn physical problems into mathematical problems to solve them. Besides, we never say what the phase is at a certain moment, only compare whether the phase is the same or not.