A horizontal cylindrical tank is half full of water ( refractive index `=(4)/(3))` . The space above the water is filled with a light liquid of unknown refractive index `(mu)`. A small laser source (s) can move along the curved bottom of the cylinder and aims a light beam towards the centre of the cylinder. The time needed by the laser beam to travel from the source to the rim of the cylinder depends on position `(theta)` of the source as shown in the graph. Find `mu`, it is given that `sin theta_(0) = (5)/(6)`.