Interactive learningware 27.1 provides some pertinent background information for this problem. two parallel slits are illuminated by light composed of two wavelengths. one wavelength is λa = 627nm. the other wavelength is λb and is unknown. on a viewing screen, the light with wavelength λa = 627nm produces its third-order bright fringe at the same place where the light with wavelength λb produces its fourth dark fringe. the fringes are counted relative to the central or zeroth-order bright fringe. what is the unknown wavelength?