In class today we talked about light.  I learned that in order to see something, light must bounce off the object and hit your eye.  So if something is hidden behind something else, the reason you cannot see it is because the other object blocks the light from hitting your eye.  Objects that block light are called opaque.  Normally, you cannot see the beam of a laser like the one in the picture above.  However, If one shoots it through water or through a cloud of dust, the beam becomes visible.  The reason for this is because the particles reflect light back into your eye allowing you to see the beam.