An electromagnetic wave is slowed down if?

Answer 1

Well, to be honest, electromagnetic wave itself is never slowed down by anything and travels uniformly at the speed of light, c. However, when you look at macroscopic level, it is possible to slow down a combination of many many electromagnetic waves if they travel though a dense medium. The denser the medium, the more 'radiation' is slowed down. So slowing down process occurs when 'radiation' travels from less dense to more dense medium (the closed atoms are packed together, the more they absorb and reemit electromagnetic waves).

Answer 2

An electromagnetic wave is slowed down if it travels through a medium with a higher refractive index than the medium it originated from. This occurs because the speed of light is slower in materials with higher refractive indices due to interactions between the light wave and the atoms in the material.