According to Eq. (4), for incidence from water (n 1 ≈ 1.333) ‍ to air (n 2 ≈ 1), ‍ we have ‍ θ c ≈ 48.6°, ‍ whereas for incidence from common glass or acrylic (n 1 ≈ 1.50) ‍ to air (n 2 ≈ 1), ‍ we have ‍ θ c ≈ 41.8°.