Let K and K′ be the maximum kinetic energy of photoelectrons for incident light of frequency υ and 2υ respectively. According to Einstein's photoelectric equation, K=hυ−E0 .......(i) and K′=h(2υ)−E0 .....(ii) =2hυ−E0=hυ+hυ−E0 K′=hυ+K [using (i)]