# Do materials have a refractive index for radio waves?

• I
• bbbl67

#### bbbl67

It's been stated that the index of refraction of materials varies with frequency throughout the EM spectrum. What are the index of refraction for various materials in the radio frequency?

The index of refraction (relative to vacuum) is the square root of the relative permittivity. For example, polystyrene, relative permittivity = 2.6 so refractive index relative to vacuum = sqrt 2.6 = 1.6 measured at 1 MHz.
Water, relative permittivity = 78 so index of refraction = sqrt 78 = 8.8 at 1 MHz.

• • bbbl67 and Delta2
It's been stated that the index of refraction of materials varies with frequency throughout the EM spectrum. What are the index of refraction for various materials in the radio frequency?
Also, can you please say what your target application is? What range of "radio" frequencies are you interested in? What kind of materials are you interested in? The better that you can ask your question, the better we can try to help you figure out the answers. Thanks.

• Delta2
The index of refraction (relative to vacuum) is the square root of the relative permittivity. For example, polystyrene, relative permittivity = 2.6 so refractive index relative to vacuum = sqrt 2.6 = 1.6 measured at 1 MHz.
Water, relative permittivity = 78 so index of refraction = sqrt 78 = 8.8 at 1 MHz.
Are radio refractive indexes just directly computed from their electrical permittivity? Optical refractive indexes don't seem to have such a straightforward relationship.

They do have but you need to use the permittivity value for the optical frequency and not the DC (or low frequency) one.

• bbbl67, berkeman and vanhees71