Refraction in an aquarium. This large tank has an acrylic (PMMA) wall that is 8.5 centimetres thick. However, the water level looks as though the acrylic is just a few millimetres thick. The two large Napoleon wrasse (Cheilinus undulatus) are, in fact, just one. From the angle of view the fish can be seen through the water with a refractive index of 1.33, while the refractive index of the acrylic is 1.49. Light reflected from the fish is refracted by the water, then by the acrylic, and then by air to the observer's eye. Photographed in La Rochelle, France.