Answer:
4.55 minute
Explanation:
51 km = 51000 m
the ray of light travels due east 51000 m , bounces back from reflecting mirrors . The reflected light gets focused on a point 135 m south . Let the angle between incident light and reflected light be θ.
tanθ = 135 / 51000
θ = .1516 degree
= .1516 x 60 minute
= 9.09 minute
angle between incident ray ( east - west ) and normal = 9.09 / 2
= 4.55 minute Ans