What in the world is constructive and destructive interference?
When two light waves intersect and thus interact with each other, interference occurs. Using water waves as an analogy, two crests (high points on the waves) or two troughs (low points) at the same place constructively interfere, adding together to produce a higher crest and a lower trough. Where a crest of one wave, however, meets a trough of another wave, there is a mutual cancellation or destructive interference. Natural interference occurs in oil slicks, producing colored patterns as the constructive interference of one wavelength occurs where other wavelengths destructively interfere.
