How does the speed of waves behave when they move from one medium to another?
speed up/slow down
1. from a vacuum to air?
2. from a solid to a liquid?
3. from a liquid to a solid?

It is simple. The higher the density, the slower the wave. If you are talking about waves that are on the Electromagnetic Spectrum (radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, gamma rays), then the speed in a vacuum is the same as the speed in the air. A solid is more dense than a liquid, so the wave will travel slower through a solid, and then vice-versa.

Hope this helps buddy :-)

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