If wavelength is known, the amount of energy in a wavelength or "packet" can be calculated. A packet (quantum) of radiation with a length of 500 nm contains 4.0 X 10^-12 ergs, or 57 kcal per mole. As wavelength decreases, energy per quantum increases, accounting for the penetration of X-rays, or the damage from nuclear radiation. It is this "quantum" energy that operates photosynthesis in green plants to produce food.