The energy E of a photon equals hυ, where h is Planck’s constant and υ denotes the light’s frequency. This alternatively is written E = ℏω, where ℏ = h/(2π) is the reduced Planck constant and ω denotes angular frequency. Its approximate value is 6.6×10−34 Js. The constant appears in Schrödinger’s equation and Heisenberg’s Uncertainty Principle.