Final answer:
Standing waves are formed by the superposition of two or more waves moving in opposite directions. They can occur in an imperfectly matched transmission line due to reflections caused by impedance mismatch.
Step-by-step explanation:
Standing waves are formed by the superposition of two or more waves moving in opposite directions. They appear to stand in place, vibrating, because the waves move through each other and their disturbances add as they go by. If the two waves have the same amplitude and wavelength, they alternate between constructive and destructive interference. Standing waves can occur in an imperfectly matched transmission line when there is a mismatch in impedance between the source and the transmission line. This mismatch causes reflections, which can result in the formation of standing waves.