Intensity of light = K units Phase difference corresponding to λ is 2π and phase difference corresponding to λ/3 is 2π/3. Now, using the formula of intensity,               I = I1+I2+2I1I2 cos ϕ Let, I1 = I2 = I0                       In the first case, when path difference is λ.              K = I0+I0+2I0 cos 2π = 4I0 In the second case, when path difference is λ3             K  = I0+I0+2I0cos2π3                 = I0+I0-2I012 = I0 Now, K K = I04I0 =14  =  K  = K4 Therefore, intensity is one-fourth of it s initial value when path difference is λ/3.                       .