Which type of electromagnetic wave if any travels through space at the slowest speed?

Answer 1

The refractive index of air at standard temperature and pressure is 1.00029. Outer space is much more rarefied and so the index is closer to 1 but it is still denser than a total vacuum and so the index is greater than 1. This means that electromagnetic waves with smaller frequencies will travel slower. Thus, blue light will travel slower than red.

Answer 2

All electromagnetic waves travel at the same speed through a vacuum - the so-called "speed of light", which is exactly 299,792,458 meters per second, by definition.

Answer 3

All electromagnetic waves travel at the same speed in a vacuum, which is the speed of light, approximately 299,792 kilometers per second.