In a YDSE set up, sodium light (λ=5890A0)  illuminates the slits, where the central 2.3 cm part of the screen just contains 21 brights. On replacing the sodium light by blue light, the central 2.8 cm part of the screen just contains 31brights. The wavelength of blue light is :

Since the central bright fringe is flanked by dark and bright fringes on either side. ∴  20 fringes occupy a length of 2.3 cm, initially,  ∴ Fringe width (β1,say)=2.3cm20 After, the sodium light is replaced by blue light, the new fringe width will be,β2=2.8cm30   Since β=λDd  ∴β∝λ                 Or,          β1β2=λsodium lightλblue light                 Or,          (2.3cm20)(302.80cm)=5890A0λblue light                 λblue light=4780A0 .