Question

At what angle above the horizon is the sun if light from it is completely polarized upon reflection from water

Answer 1

In order to be completely polarized, the incident angle (the angle with perpendicular to the surface) should be equal to Brewster's angle:

$$\theta_B = \arctan\left( \dfrac{n_2}{n_1}\right)$$

where $n_2 = 1.33$ is the refractive index of water and $n_1 = 1$ is the refractive index of air.

Thus, obtain:

$$\theta_B = \arctan\left( \dfrac{1.33}{1}\right) \approx 53.1\degree$$

The angle abov the horizon is then:

$$\alpha = 90\degree - \theta_B = 36.9\degree$$

Answer. $36.9\degree$.