The medium of a wave on a spring is the spring itself. The spring is what is vibrating and causing the wave to propagate. The wave is a mechanical wave, which means that it requires a medium to travel through. In this case, the medium is the spring. The wave can be caused by a variety of things, such as plucking the spring or hitting it with an object. Once the spring is vibrating, the wave will travel along the spring until it reaches the end.