Young's experiment is perfonned with light from excited helium atoms ( = 502 om). Fringes are measured

Question:

Young's experiment is perfonned with light from excited helium atoms (λ = 502 om). Fringes are measured carefully on a screen 1.20 m away from the double slit, and the center of the 20th fringe (not counting the central bright fringe) is found to be 10.6 mm from the center of the central bright fringe. What is the separation of the two slits?