We see a lot of colorful things around us. What exactly is the color and what make the things exhibit these colors? We know that the color we see is the visible region of the electromagnetic spectrum. We also know that matter can absorb the electromagnetic radiation of different energy (or wavelengths). The region of electromagnetic energy that is not absorbed is simply reflected back or gets transmitted through the matter. The colored compounds are colored because they absorb the visible light. The color that is perceived is called the complement color to the absorbed wavelength and is represented by a color wheel (Figure 4.1).

Figure 4.1 A simplified color wheel showing complementary colors. Green is interesting as it can arise from the absorption of radiation to either end of the visible spectrum.

Absorption of ultraviolet (UV) and visible radiation is one of the most routinely used analytical tools in life sciences research. The simplest application of UV/Visible radiation is to quantify the amount of a substance present in a solution. UV region of electromagnetic radiation encompasses the wavelengths ranging from ~10 nm – ~400 nm while visible region encompasses the wavelengths from ~400 nm – ~780 nm. For the sake of convenience in discussing the observations, UV region is loosely divided into near UV (wavelength region nearer to the visible region, λ ~ 250 nm – 400 nm), far UV region (wavelength region farther to the visible region, λ ~ 190 nm – 250 nm) and vacuum UV region (λ< 190 nm). The wavelength ranges defined for these regions are not strict and people use slightly different ranges to define these regions. We shall, however, stick to the wavelengths defined here . As has been discussed in the previous lecture, the absorption of UV and visible light is through the transition of an electron in the molecule from lower to a higher energy molecular orbital. The various electronic transitions observed in organic compound are shown in Figure 4.2.