I know that in a vacuum, speed of light is constant. My question is more about the speed of light in a material like air. Dispersion of light in a prism tells us that the speed of light or the material index depends on the wavelength ( or frequency which is constant ) so I thought that air induces also a dispersion in EM waves. If true, I want to know if there is a relation between velocity of EM waves and frequency in air. my aim is to calculate speed of light for different frequencies (radio frequencies) in air.