If the sample compound does not absorb light of of a given wavelength, I = I₀. However, if the sample compound absorbs light then I is less than I₀, and this difference may be plotted on a graph versus wavelength, as shown on the right. Absorption may be presented as transmittance (T = I/ I₀) or absorbance (A= log I₀/I). If no absorption has occurred, T = 1.0 and A= 0. Most spectrometers display absorbance on the vertical axis, and the commonly observed range is from 0 (100% transmittance) to 2 (1% transmittance). The wavelength of maximum absorbance is a characteristic value, designated as λ_max.

We have learnt basic theory and instrumentation of UV – Visible spectrophotometry