In Young’s double-slit experiment net intensity of light at a point on screen is $I_{\text{net}}$ = $I_1+I_2+2\sqrt{I_1I_2}$ cos ϕ For $I_1$ = $I_2$ = I, $I_{\text{net}}$ = 2I + 2I cos ϕ Relation between path difference and phase difference is Δϕ = $\frac{2\pi}{\lambda}$ Δx For path difference λ, phase difference, Δϕ = $\frac{2\pi}{\lambda}$ λ = 2π $I_{\text{net}}$ = K = 2I + 2I cos 2π or K = 4I ... (i) For a path difference λ/3, phase difference Δϕ = $\frac{2\pi}{\lambda}$ Δx ⇒ ϕ = $\frac{2\pi}{\lambda}\left(\frac{\lambda}{3}\right)$ = $\frac{2\pi}{3}$ Now the intensity, $I_{\text{net}}$ = 2I + 2I cos $\frac{2\pi}{3}$ $I_{\text{net}}$ = 2I - 2I sin 30° ⇒ $I_{\text{net}}$ = I = $\frac{K}{4}$