Question: principle of electronic transition in uv visible spectroscopy

title: Ultraviolet–visible spectroscopy
text: In a double - beam instrument , the light is split into two beams before it reaches the sample . One beam is used as the reference ; the other beam passes through the sample . The reference beam intensity is taken as 100 % Transmission ( or 0 Absorbance ) , and the measurement displayed is the ratio of the two beam intensities . Some double - beam instruments have two detectors ( photodiodes ) , and the sample and reference beam are measured at the same time . In other instruments , the two beams pass through a beam chopper , which blocks one beam at a time . The detector alternates between measuring the sample beam and the reference beam in synchronism with the chopper . There may also be one or more dark intervals in the chopper cycle . In this case , the measured beam intensities may be corrected by subtracting the intensity measured in the dark interval before the ratio is taken .