An opaque cylindrical tank with an open top has a diameter o

An opaque cylindrical tank with an open top has a diameter of 3.30 m and is completely filled with water. When the afternoon Sun reaches an angle of 20.8° above the horizon, sunlight ceases to illuminate the bottom of the tank. How deep is the tank?
m

Solution

90 - 20.8 = 69.2 deg

n₁ sin theta₁ = n₂ sin theta₂

1 * sin 69.2 = 1.33 * sin theta₂

theta₂ = 44.67 deg

tan theta₂ = 3.30 / h

h = 3.30 / tan theta₂ = 3.30 / tan 44.67

h = 3.34 m