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The resonance lines of \ion{Si}{4} formed at $\lambda$1394 and 1403 {\AA} arethe most critical for the diagnostics of the solar transition region in theobservations of the Interface Region Imaging Spectrograph (IRIS). Studying theintensity ratios of these lines (1394{\AA}/1403{\AA}), which under opticallythin condition is predicted to be two, helps us to diagnose the opticalthickness of the plasma being observed. Here we study the evolution of thedistribution of intensity ratios in 31 IRIS rasters recorded for four daysduring the emergence of an active region. We found that during the early phaseof the development, the majority of the pixels show intensity ratios smallerthan two. However, as the active region evolves, more and more pixels show theratios closer to two. Besides, there are a substantial number of pixels withratio values larger than 2. At the evolved stage of the active region, thepixels with ratios smaller than two were located on the periphery, whereasthose with values larger than 2 were in the core. However, for quiet Sunregions, the obtained intensity ratios were close to two irrespective of thelocation on the disk. Our findings suggest that the \ion{Si}{4} lines observedin active regions are affected by the opacity during the early phase of theflux emergence. The results obtained here could have important implications forthe modelling of the solar atmosphere, including the initial stage of theemergence of an active region as well as quiet Sun.

On the ratios of \ion{Si}{4} lines ($\lambda$1394/$\lambda$1403) in an emerging flux region