Acrylic is frequently used in optics, where structural properties are a secondary consideration. However, for some structures it is very important that it should withstand pre-defined static and dynamic effects – examples are walkways and aquariums. Acrylic structures are more complex than steel, for example, because time and temperatures influence its character. Acrylic is viscoelastic and steel is linear elastic-perfectly plastic. This means that the strength of acrylic will change depending on how long it must bear a load and at what temperature. It also loses strength when exposed to UV rays and especially when exposed to organic solvents. For this reason, it is essential that acrylic used in higher safety groupings is regularly inspected.