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Hi Sunmeet, with “bands”, in this case, we mean absorption features (absorbed radiation in a specific wavelength region, causing a “depression” in the reflectance spectrum). I see that this can be misleading in the context of the MOOC but this is how it is referred to in the literature. The electronic transition consumes more energy while vibrational processes are less energy consuming, therefore, electronic transitions tend to cause larger absorption bands, whereas vibrational transitions are narrower. There are some more details on causes for specific bands of specific materials (mostly minerals) e.g. in Clark 1999 and Hunt 1977 (full references given in the reference list 3.6 Spectral reflectance of surface materials
Geology/Minerals). I hope that helps. :). Arlena