Coherent versus jumbled light

The first complication is that light sources usually do not emit coherent light, a term that means the wavefronts are perfectly aligned in time and space. A laser is an exceptional case that indeed produces coherent light. It emits parallel waves of a constant wavelength that are also synchronized in time so that their peaks align as they propagate. Common light sources, such as light bulbs and the sun, instead emit a jumble of waves that have various wavelengths and do not have their peaks aligned.

Figure 4.5: Visible light spectrum corresponds to the range of electromagnetic waves that have wavelengths between 400nm and 700nm. (Figure by David Eccles for Wikipedia.)