The repulsion of free electrons on the surface of CuO NPs results in the formation of the maximum peak shown in Figure , with light intensity at the wavelength observed at its maximum absorbance . According to El-Trass et al. and Velsankar et al. , there is a proportional relationship between the content of CuO NPs present in solution with the wavelength at the maximum observed absorbance, where a value greater than 300 nm suggests a high amount of CuO NPs formed per volume of solution. Furthermore, the shift in the band of the UV-Vis spectrum between 250-800 nm is a result of the morphology, size, and state of aggregation of the NPs .