A photoelectric material having work–function ϕ0 is illuminated with light of wavelength λ(λ< hc/ϕ0 ). The fastest photoelectron has a de–Broglie wavelength λd. A change in wavelength of the incident light by Δ λ result in a change Δ λd in λd. Then the ratio Δ λd/ Δ λ is proportional to