When the angle of incidence is equal to the critical angle, the ray of light grazes the surface. Hence, the light will appear to emerge out from a circle whose edges make angle i c with the vertical.
The area of surface of water through which light will come out, a = πr2
µ = 1/sinic ∴ sinic = 1/µ = 1/1.33 = 0.752
∴ ic = 48.76°. tanic = AB/AO ⇒ AB = AO tanic
r = 80 x tan 48.76° = 91.2cm.
∴ Area, a = πr2 = π × (0.912)2 = 2.61m2