Assume a hypothetical 3 media/ 1 layer structure with the following indices of refraction: $$n_1 = 2, n_2 = 2+i0.5, n_3 = 1$$ where the thickness of the layer is 100 nm and wavelength = 1000 nm.
Using Fresnel equations to derive the total transmittance and reflectance I get that about ~38% of incident power is absorbed in the layer.
If we now add another boundary, so that the structure now looks like: $$n_1 = 2, n_2 = 2+i0.5, n_3 = 1, n_4 = 3$$
The absorption in the first layer as a function of the second layer thickness now looks like:
My question is how is that the absorption now goes below the ~38%?
I understand that when deriving the transmission and reflectance I am summing up the electric field amplitude, some of which might cancel due to the reflections in the second layer but intuitively this still doesn't make sense.