Duane–Hunt law, in atomic physics, the relationship between the voltage (V ) applied to an X-ray tube and the maximum frequency ν of the X rays emitted from the target. It is named after the American physicists William Duane and Franklin Hunt. The relationship is expressed as ν = Ve/h, in which e is the charge of the electron and h is Planck’s constant. This law is sometimes called the inverse photoelectric equation.