We wish to examine the irradiance produced on the plane of observation in Young’s Experiment when the slits are illuminated simultaneously by two monochromatic plane waves of somewhat different frequency, E₁ and E₂. Sketch these against time, taking λ₁ = 0.8 λ₂. Now draw the product E₁E₂ (at a point-P ) against time. What can you say about its average over a relatively long interval? What does (E₁ + E₂)² look like? Compare it with E²₁ + E²₂ . Over a time that is long compared with the periods of the waves, approximate ((E₁ + E₂)²)_T.