1. The problem statement, all variables and given/known data Light of two wavelenghts L1 and L2 (L1,L2 = lambda)creates interference fringes in the Young's double slit experiment. To calculate the least distance from the central maximum where the bright fringes for both L1 and L2 coincide. Given: the distance 'd' between the two slits, and 'D' between the plane of the slits and the screen. 2. Relevant equations Distance of the nth bright fringe from the central maximum = nLD/d For the two bright fringes to overlap, n1L1 = n2L2 (since D/d is common for both wavelengths) n1 corresponds to the first wavelength n2 corresponds to the second wavelength 3. The attempt at a solution By the above I am able to get the ratio n1/n2. However, I am unable to proceed any further to determine the required distance to the first point of overlap of the bright fringes.