Answer:
When a wave crosses a boundary between two different media, such as a thin and a thick rope, its speed and wavelength change due to the difference in the properties of the two media. However, the frequency of the wave remains constant because it is determined by the source of the wave and is independent of the medium through which it travels. Since the frequency of a wave is equal to its speed divided by its wavelength (f = v/λ), if the speed of the wave changes while its frequency remains constant, its wavelength must also change to maintain this relationship.