The index of refraction for materials in radio frequencies is determined by the square root of their relative permittivity. For instance, polystyrene has a relative permittivity of 2.6, resulting in a refractive index of 1.6 at 1 MHz, while water, with a relative permittivity of 78, has an index of 8.8 at the same frequency. The discussion highlights that the index of refraction varies with frequency across the electromagnetic spectrum. Additionally, it emphasizes the importance of specifying the target application and frequency range for more tailored assistance. Understanding these refractive indices is crucial for applications involving radio waves.