Two narrow parallel slits separated by 0.850 mm are illuminated by 600-nm light, and the viewing screen is 2.80 m away from the slits.
(a) What is the phase difference between the two interfering waves on a screen at a point 2.50 mm from the central bright fringe?
(b) What is the ratio of the intensity at this point to the intensity at the center of a bright fringe?