State two conditions to obtain sustained interference of light
In Young's double slit experiment, using light of wavelength 400 nm, in terference fringes of width 'X' are obtained. The wavelength of light is enhanced to 600 nm and the separation among the slits is halved. If one wants the observed fringe width on the screen to be the same in the two cases, find the ratio of the distance among the screen and the plane of the interfering sources in the two arrangements.