For example the carbonyl group bonds contain sigma and pi electrons. These electrons may transition from bonding to antibonding levels. As light pases through a compound in solution the intensity is reduced. The longer the pathlength the more light is absorbed. Also, the higher the concentration of compound in solution the more light is absorbed.
Absorbance is read from the upper scale on the meter.
The spectrophotometer should be turned on at least 30 minutes before calibration. Adjust the wavelength to the required setting. This procedure should be made for each wavelength of interest. The spectrophotometer should remain on until all the required readings are made.