A pole of length

81.5 cm

Length of the pole= AB= 1m

Half of the length of the pole is under water and half is above the water surface. The point A of the pole is above the water and the light ray will first strike the water surface at the angle 45° normal to the surface and then the light rays will refract into the surface. The angle of refraction is r.

Now, the shadow length BA’ = BD + A’D = 0.5 + 0.5 tan r

Now using Snell’s law,

So, shadow length = (0.5) (1 + 0.6235) = 81.2 cm