Question

The index of refraction of water is 1.33. If you (or a fish) were under calm water swimming in the daytime, looking upward you would see a bright circle of light above you at the surface. The circle is because of total internal reflection, and the boundary is where light from below is totally reflected if it is beyond the boundary. What would you see just at the inside edge, where there is the light of the sky coming in?

		An image of yourself, because of the law of reflection.
		The horizon of the pool or lake, because rays in the air at that position are horizontal and parallel to the surface.
		Directly overhead, probably the Sun, because the rays in air at that point are perpendicular to the surface.
		The bottom of the lake, reflected in the water., because of total reflection.

Answer 1

You must be seeing the bottom of the lake reflected by the total internal reflection. If the incident angle of the bottom of the lake at the surface of the water is more than a critical angle, the light will be totally reflected back and you will see a bright circle of light.
The total internal reflection happens when a ray of light enters a medium of lower refractive index from a medium of higher refractive index. in this case, the light enters from the water of high refractive index to the air of lower refractive index. So there must be a critical angle at which the total internal reflection happens.