A beam of light consisting of two wavelengths 650nm and 520nm is used to obtain interference fringes in Young's double slit experiment. The distance between slits is 2 mm and between the plane of slits and screen is 120 cm. The least distance from the central maximum where the bright hinges due to both wavelength coincide is

let the nth bright fringe of wavelength λm and m th Bright fringe of wavelength λm coincide at a distance x n from central maximum .then xn=nλnD2d=mλmD2d or mn=λnλm=650nm520nm=54 So, least integral values of m and n satisfying above requhemeDt are m = 5andn= 4 Smallest value of xn, is given by  xnmin=nλnDd=4×650×10−9×1.202×10−3 = 1.56 mm