1) In a double-slit experiment, two parallel slits are illuminated first by light of wavelength 460nm, and then by light of unknown wavelength. The third-order (m = 3) dark fringe resulting from the known wavelength of light falls in the same place on the screen as the second-order (m = 2) bright fringe from the unknown wavelength. What is the unknown wavelength? 2) What is the minimum thickness of a soap bubble needed in order for the reflected light from the outer and inner surfaces to constructively interfere? The light incident on the film has a wavelength of 655nm. Assume that the index of refraction for the soap film is 1.35. Help would be really appreciated, I've spent over an hour on these two problems and can't figure them out at all.