The photoelectric effect, which was first explained by Einstein in 1905 and earned him a Nobel Prize, refers to the observations that photons of sufficient energy can cause electrons to be removed from a metal and cause an electric current to flow. The minimum energy needed to achieve this is the photoelectric work function, Φ. It can be determined by measuring the excess kinetic energy of the electrons emitted from a surface exposed to light of energy hv such that Φ = h l J - 1 2 m e v 2 $$ \Phi = h_{{lJ}} - \frac{1}{2}m_{e} v^{2} $$ https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781315381381/fbc2c269-f5ee-4e6a-83ad-0fb13faed8c5/content/math8_1.tif"/>