The principle of superposition
Electric fields created by different sources, e.g., by two or more point charges, simply add together as vectors. Similarly magnetic fields created by different sources, e.g., by two or more current-carrying wires, also add together as vectors. This superposition principle applies to all electric and magnetic fields, including those comprising electromagnetic waves created by different sources. If the E vectors point approximately in the same direction at a given instant of time, the result of adding the vectors will be a sum that is larger than its parts: this is known as constructive interference. If on the other hand, the E vectors point approximately in opposite directions, the result will be smaller than its parts, which is known as destructive interference.
Superposition works for other types of waves also. For example, when small-amplitude waves on the surface of a liquid pass each other, the superposition of two wave crests passing each other creates an extra-high crest; while a crest passing a trough creates a flat spot. (Large-amplitude waves in the ocean are a more complicated story: when two of them meet, they can cause each other to break.) Superposition/interference effects for sound waves will be demonstrated in lecture.