7.1.6 Object-Media

From [pov:6.6.8]:
The interior statement may contain one or more media statements. Media is used to simulate suspended particles such as smoke, haze, or dust. Or visible gasses such as steam or fire and explosions. When used with an object interior, the effect is constrained by the object's shape. The calculations begin when the ray enters an object and ends when it leaves the object. Any interior media patterns are totally independent of the texture.