If wavelength changes by Δλ then change in propagation vector is:
A photoelectric material having work-function ϕ0 is illuminated with light of wavelength λ(λ<hcϕ0) . The fastest photoelectron has a de Broglie wavelength λd . A change is the wavelength of the incident light by Δλ result in change Δλdinλd .then the ratio Δλd/Δλ is proportional to