Asked by: Sam Lipley, Braunton
A large public aquarium, like the one pictured in Okinawa, Japan, can contain 7.5 million litres. That’s more than 7,500 tonnes of water, held back by a single window 22.5m across. Glass isn’t actually very suitable for a window this large. It is brittle, so it needs to be very thick to stop it from shattering and at such thicknesses, the high refractive index of glass distorts the view of the fish in the tank.
Instead, polymethylmethacrylate (PMMA) is used. This is also known simply as acrylic or by its various trade names, such as Plexiglass and Lucite. PMMA is half the density of glass and has twice the tensile strength. It also flexes and has a lower refractive index. This allows the aquarium window to be thinner and give a clearer view. Even so, the aquarium window below is 60cm thick.