where formula_2 is the photon's energy, formula_3 is the Plank constant, and formula_4 is the frequency of the light associated with the photon. This frequency, formula_4, is typically measured in cycles per second, or equivalently, in Hz. The quantum energy of different photons is often used in cameras, and other machines that use visible and higher than visible radiation. This because these photons are energetic enough to ionize atoms. 