Interference was observed in interference chamber when air was present, now the chamber is evacuated and if the same light is used, a careful observer will see
(1) No interference
(2) Interference with bright bands
(3) Interference with dark bands
(4) Interference in which width of the fringe will be slightly increased
(4) The refractive index of air is slightly more than 1. When chamber is evacuated, refractive index decreases and hence the wavelength increases and fringe width also increases.