Given, mixture of light consist of two wavelengths (i) Known wavelength, λ1=590nm (ii) Unknown wavelength, λ2=? Since, central maxima of both light coincides. Also, the third bright fringe of light of wavelength λ1, coincides with the fourth bright fringe of light of wavelength λ2. Therefore, n1λ1=n2λ2 Here, n1(known)=3(unknown),n2(unknown)=4 Put the values, 3×590=4×λ2 ⇒ λ2=4425nm