We address the issue of anomalous image flux ratios seen in the double-image gravitational lens JVAS B0218+357 .
From the multi-frequency observations presented in a recent study ( ) and several previous observations made by other authors , the anomaly is well-established in that the image flux-density ratio ( A/B ) decreases from 3.9 to 2.0 over the observed frequency range from 15 GHz to 1.65 GHz .
In , the authors investigated whether an interplay between a frequency-dependent structure of the background radio-source and a gradient in the relative image-magnification can explain away the anomaly .
Insufficient shifts in the image centroids with frequency led them to discard the above effect as the cause of the anomaly .

In this paper , we first take this analysis further by evaluating the combined effect of the background source extension and magnification gradients in the lens plane in more detail .
This is done by making a direct use of the observed VLBI flux-distributions for each image to estimate the image flux-density ratios at different frequencies from a lens-model .
As a result of this investigation , this mechanism does not account for the anomaly .
Following this , we analyze the effects of mechanisms which are non-gravitational in nature on the image flux ratios in B0218+357 .
These are free-free absorption and scattering , and are assumed to occur under the hypothesis of a molecular cloud residing in the lens galaxy along the line-of-sight to image A .
We show that free-free absorption due to an H ii region covering the entire structure of image A at 1.65 GHz can explain the image flux ratio anomaly .
We also discuss whether H ii regions with physical parameters as derived from our analysis are consistent with those observed in Galactic and extragalactic H ii regions .