 When "classical" (coherent or thermal) light is incident on a photographic plate or CCD, the average number of "hits", "dots", or "clicks" per unit time that result is approximately proportional to the square of the electromagnetic fields of the light.By "formal analogy", the wavefunction of a material particle can be used to find the probability density by taking its absolute-value squared.Unlike electromagnetic fields, quantum-mechanical wavefunctions are complex.