When taking wide angle photos underwater we observe a remarkable effect: if we look up in quiet water, it appears that we can look at the world above water through a circular window.
The reason is found in Snell’s law, which states that light is refracted when going from one medium to another medium with different ‘refraction index’. In our case light goes from air to water. Light propagates not in a straigt line, but is refracted.
At the surface its is bent as shown below: going from air to water the angle with the normal to the water surface decreases.
Looking straigt up we see what is above us. Looking under an small angle we can still look above water. However, when the angle increases and goes beyond the critical angle, we can no longer look above water: light is no longer refracted, but the reflected: the surface acts as a mirror.
In the above example we look at the bottom of a water lily flower. Clearly the water lily is beyond the window, in the total reflection region, as can be seen by teh reflection of the leaves.
The crown of this flower can be seen at the border of the window, when looking outside the water. It seems separated from the flower under water. The reason again is refraction. This is shown in next figure.
The almost horizontal rays (large angle with the normal of the water surface) of light are bent towards the diver (smaller angle with the normal). To the diver it appears that the flower is positioned much higher than it realy is: this is the virtual image. It appears to be at an other position than the real image, as is apparent on the photograph of the water lily.
Below two more examples with the window, a real window of opportunities for the photographer.