Einstein's Photoelectric Equation is based on the Conservation of Energy: $\text{Energy of incident photon}=\text{Work Function}+\text{Max Kinetic Energy of electron}$ $hf=\Phi +K_{max}$ $hf=h{f}_0+K_{max}$ $K_{max}=hf-h{f}_0=h(f-f_0)$ where $h$ is Planck's constant, $f$ is incident frequency, and $f_0$ is threshold frequency.