What determines the speed of electromagnetic waves through a medium?

The speed of electromagnetic waves in a perfect vacuum is a universal constant, approximately 300 million meters per second. However, when these waves travel through a physical medium like air, glass, or water, they slow down. The ratio of the speed of light in a vacuum to the speed of light in a specific medium is known as that medium's index of refraction. Therefore, the index of refraction directly dictates how fast the electromagnetic wave can propagate through that material. Distractors like resistance and reactance apply to alternating electric currents in circuits, not the propagation speed of waves in a medium. Birefringence and evanescence refer to specific optical polarization and boundary phenomena, not the general speed of the wave.