Liquid tank, viewing device for under-liquid observation, and optical film

A liquid tank having a transparent wall, comprising,

• a first moth-eye layer having a first surface in direct contact with an internal surface of the transparent wall and a second surface having a moth-eye structure opposite the internal surface of the transparent wall,
• a protective layer on the moth-eye structure, the protective layer having a refractive index substantially equivalent to that of a liquid to be stored in the tank,
• wherein the protective layer is configured to simultaneously contact the liquid and air above the liquid, and
• a second moth-eye layer having a first surface in direct contact with an external surface of the transparent wall and a second surface having a moth-eye structure opposite the external surface of the transparent wall, wherein the second moth-eye layer is disposed in a region facing the first moth-eye layer and wherein the moth-eye structure of the second moth-eye layer is directly exposed to air, wherein
• the refractive index of the protective layer is different from that of the air.