The fourth-order bright fringe is observed at point P on the screen when a double-slit experiment is performed in air using light of wavelength 600nm. The entire apparatus (slits and screen fixed) is now immersed in a transparent liquid of refractive index 1.5 without changing any distances.
(a) Calculate the new wavelength of the light in the liquid.
(b) State whether point P will now correspond to a bright fringe, a dark fringe, or neither. Give a brief justification.