When the light moving through the smoke or dust will see a beam straight and can penetrate through the object. Object that allows light moving through the straight away. We call this object that the transparent objects such as glass, air, water, etc.
If the light moving through the object of some kind, then the distribution of light surrounding the moving light non-linear. We call that object. Translucent objects, such as glass, wax paper, plastic ceiling, etc.
The objects that allow moving light pass. We called opaque object, such as the wall of concrete thick cardboard etc. opaque object will reflect some absorption and some shadows.