. A photoelectric material having work-function ϕ0 is illuminated with light of wavelength The fastest

A photoelectric material having work-function ϕ₀ is illuminated with light of wavelength The fastest 

photoelectron has a de Broglie wavelength λ_d. A change in wavelength of the incident light by ∆λ results in a change ∆λ_d in λ_d. Then the ratio ∆λ_d/∆λ is proportional to