calculate the critical angle for a water and air boundary:

sin qc = 1/1.33 = 0.75

Þ qc = sin^-1 (0.75) = 48.6^o

To work out r we need some trigonometry

r = 1.8 ´ tan 48.8 = 1.8 ´ 1.13 = 2.04 m