1, first of all. That is, there is no energy at the trough of the light wave, then the energy is zero, then the lowest trough of the light intensity distribution map coincides with the X axis, and then there is no waveform below the X axis.

2. second. Even the waveforms of water waves are the same as those of light waves. Taking the lowest channel of the trough as the zero point of energy, then the lowest point of the trough and the zero point of the ordinate will turn again, and there will be no waveform below the X axis.

3. finally. The waveforms of mechanical waves such as water waves are drawn in this way. The above is the waveform drawing method of calibration signal.