The larger the incident angle (relative to the surface normal), the larger the angle of the refracted beam (relative to the surface normal). The largest possible incident angle that is physical possible is just barely under 90 (i.e., 89.99999 ; but we will just use 90 for our calculation). From Snell's Law, we have : n1sin 1 = n2sin 2 plugging in n1 = 1.00 for air, n2 = 1.33 for water, and 1 = 90 , we get: sin 2 = (n1/n2)sin 1 = (1.00 / 1.33) 1 = 0.7519 2 = sin-1 (0.7519) = 48.75