The optical energy band gap of a material can be calculated from its absorption spectrum obtained from a UV-Vis spectrophotometer. The band gap is related to the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of a material. This energy difference is equal to the energy of the photon required to promote an electron from the valence band (HOMO) to the conduction band (LUMO).

In a UV-Vis absorption spectrum, the band gap energy can be determined from the onset of absorption, which is where the absorption coefficient starts to increase sharply. The onset of absorption corresponds to the minimum energy required to excite an electron from the valence band to the conduction band. The band gap energy can be calculated from the onset of absorption using the Tauc plot method, which involves plotting (αhν)n versus hν, where α is the absorption coefficient, hν is the photon energy, and n is a parameter that depends on the nature of the electronic transition.

On the other hand, the reflectance spectrum can also provide information about the band gap of a material. The band gap energy can be calculated from the reflectance spectrum using the Kubelka-Munk theory, which relates the absorbance and reflectance of a material to its optical properties. However, the absorption spectrum obtained from a UV-Vis spectrophotometer is typically easier to measure and provides more information about the electronic transitions in the material.