The Frank–Tamm formula describes the amount of energy E emitted from Cherenkov radiation, per unit length traveled x and per frequency ω ω . μ μ ( ω ω ) is the permeability and n ( ω ω ) is the index of refraction of the material the charged particle moves through. q is the electric charge of the particle, v is the speed of the particle, and c is the speed of light in vacuum.