A planar structure of length L and width W is made of two different optical media of refractive indices n1=1.5 and n2=1.44 as shown in figure. If  L>>W, a ray entering from end AB will emerge from end CD only if the total internal reflection condition is met inside the structure. For  L=9.6m, if the incident angle θ is varied, the maximum time taken by a ray to exit the plane CD is t×10−9 s, where t is___