(1) energy Explanation: The Planck's constant (h) can be defined as a pro-portionality constant that relates the energy (E) of a photon to the frequency (?) of its associated electro-magnetic wave. Mathematically, Planck's Constant (h) = Energy (E)/frequency (?). The Planck constant is named after Max Planck, the instigator of quantum theory, who discovered it in 1900.