A lighthouse is 120 feet (36.6 meters) high and the light has a nominal range of 18 miles. Your height of eye is 42 feet (12.8). If the visibility is 11 miles, approximately how far off the light will you be when the light becomes visible?

Correct Answer: Option C — 19.0 miles

Calculate the geographic range: 1.17 * sqrt(120) + 1.17 * sqrt(42) = 12.8 + 7.6 = 20.4 miles. Since the visibility is only 11 miles, the light is visibility-limited. However, the question asks for the distance at which it becomes visible, which is the lesser of geographic or luminous range, resulting in 19 miles.