On the Absorption Distance - with a value of 40 cm, after traveling 40 cm through the volume, the ray will have the color specified in the Absorption Color; between 0 and 40 cm it will already be graduating from nothing to the Absorption Color though, so will not be "clear" other than at a distance of 0. Raising the Distance well above the 40cm will make it fade out more. As an alternative, or as an additional effect, you can vary the Absorption Color through a Corona Distance map (Max) or Proximal map (C4D), so that at a certain distance from a certain object the Absorption color is pure white (which counts as zero absorption, that is, no effect).