Thanks it makes a lot more sense now!
The sign of the reflected wave (up or down) depends on the mediums that it travels from & to. A wave with a positive amplitude that is traveling from a medium with light density (air) to a heavier density medium (water) will reflect with a negative amplitude.
This action is analogous to a string tied onto a post. The wave will be reflected with opposite amplitude.
Two springs that are connected will follow the same principle at the boundary between them.
A pulse sent from a light density spring to a heavy density spring will reflect back with an opposite amplitude.
A pulse sent from a thick density spring to a light density spring will reflect back with the same amplitude.
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