A flat piece of glass covers the top of a vertical cylinder that is completely filled with water. If a ray of light traveling in the glass is incident on the interface with the water at an angle of theta_a = 36.0degrees, the ray refracted into the water makes an angle of 49.1degrees with the normal to the interface.
What is the smallest value of the incident angle theta_a for which none of the ray refracts into the water?