Evolution of the main features of the computed optical absorption spectra as a function of carbon cluster size. Panel a: position of the maximum for the UV bump; the expected range for the positions of the astronomical UV bump on the extinction curve in the interstellar medium (ISM; Fitzpatrick & Massa 2005, 2007) or in circumstellar environnments (CSM; Drilling et al. 1997; Buss et al. 1989; Waelkens et al. 1995; Gavilan et al. 2017) is also reported. Panel b: position of the first electronic transition, with dashed lines drawn to guide the eyes.