We analyze the absorption coefficient of planarized silicon-on-insulator waveguides coated by close packed mono- and multilayers of colloidal PbS/CdS quantum dots (QDs). Experimental data clearly show the influence of the QDs on the waveguide absorbance around 1500 nm, where we find that QDs absorb stronger in thicker layers. To simulate the absorption coefficient of QD functionalized waveguides, the QD layer is replaced by an effective medium with a dielectric function determined by dipolar coupling between neighbouring QDs. Using the host dielectric constant εh as an adjustable parameter, excellent agreement with the experimental results is obtained. In this way, the increase in absorption cross section with layer thickness can be traced back to an increasing εh. We argue that this reflects the decreasing influence of the surroundings on the εh, which therefore evolves from an extrinsic property for monolayers to a more intrinsic film property for multilayers.