Constructive interference is when a crest from one source meets a crest from another source, the energies combine to displace the medium (the energies are additive). In other words, when the two crests meet, they produce a single amplitude equal to the sum of the two individual amplitudes. The same thing occurs when trough meets trough. When the crests of the waves line up, there is constructive interference. Often, this is described by saying the waves are “in-phase”. In the first video, we put the slinky on the ground and flicked the same way to represent the crest meeting the other crest. The same thing occurred in the second video except we flicked the other way to represent the trough meeting another trough.
Destructive interference is when a crest and trough meet the energies combine to work against each other- they tend to cancel out. The sum of two waves can be less than either wave and can even be zero. When the crests of the wave in one wave match up with the troughs of the wave in the other, the waves are said to be “out-of-phase”. In this video, the slinky is flicked 2 separate ways to represent a crest and a trough. When they meet, they