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.
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.