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

18

$n_1λ_1=n_2λ_2$

$∴n_2=\frac{n_1λ_1}{λ_2}=\frac{12×600}{400}=18$