Generally speaking, the speed of a wave depends on the medium and the wave type, so waves of the same type in the same medium attain the same speed

Question

My question is, given the above statement, why electromagnetic waves attain different speeds in media other than vacuum even though they are of the same type and propagate in the same medium?

Answer 1

The speed of a wave depends on the type of wave, on the medium but also, not always but in most cases, on the frequency.

For instance, in media other than vacuum, lower frequency (red) light waves propagate faster than higher frequency (purple) ones. This is what causes rainbows.