A small light is 13.0 cm below the surface of a liquid of refractive index 1.20. Viewed from above, the light appears to illuminate a circle on the surface of the water. What is the diameter of the circle?
The light that leaves the liquid fills a conical volume from the source below. Larger angles are totally reflected internally. The half cone angle is the angle of incidence for total reflection, which is sin^-1 (1/1.2) = 56.4 degrees
The radius of the circle is
13.0 tan56.4 = 19.6 cm and the diameter is twice that