In a Young's double slit experiment, 12 fringes are observed to be formed in a certain segment of the screen when light of wavelength 600 nm is used. If the wavelength of light is changed to 400 nm, the number of fringes observed in the same segment of the screen is given by

Fringe width, β=λDd i.e., β∝λWhen the wavelength is decreased from 600 nm to 400 run, fringe width will also decrease by a factor of 416 or 213 or the number of fringes in the same segment will increase by a factor of 312. Therefore, number of fringes observed in the same segment=12×32=18.