 For example, the molecule mercury dithizonate is a yellow-orange color in diluted solution (1*10^-5 M), and turns blue when subjected with particular wavelengths of visible light (and UV) via a conformational change, but this reaction is reversible back into the yellow "ground state".Using optical fibers as a transmission element of spectrum of burning gases it is possible to determine a chemical composition of a fuel, temperature of gases, and air-fuel ratio.The rate constant of a particular reaction can be determined by measuring the UV/Vis absorbance spectrum at specific time intervals.