Absorb the Light

Color comes from the absorption of visible light. When light shines on or through a substance some of the light is absorbed. The light that is not absorbed is reflected and this is the color we see. If only red light is absorbed we see green. If only blue light is absorbed we see orange. If all the light is reflected the color is white. If all the light is absorbed the color is black. This is why we wear white on hot days, to reflect the sunlight and stay cooler. Colored solutions contain chemicals which absorb visible light. As explained in the blog "Beer's Law" (August 24th, 2009), the amount of light absorbed is directly proportional to the concentration of the chemical. A spectrophotometer is an instrument designed to measure absorbance of light for the whole visible spectrum very accurately.Some spectrophotometers also measure light absorption in the near ultraviolet (UV) and near infrared (IR). Spectrophotometers can be double beam where the beam of light is split and passed through a sample and reference at the same time. The reference signal is then subtracted from the sample signal. A single beam spectrophotometer passes the light beam through the reference and then the sample. Scanning spectrophotometers scan the absorbance wavelengths of light exiting from the sample producing an absorption spectrum. There are basic spectrophotometers such as the Thermo Spectronic 20 or the Unico S1000 for basic absorption measurements. There are mid level spectrophotometers such as the Genesys 10 and the Unico S2100. Advanced spectrophotomers include the Beckman Coulter DU 800 and the Unico 4802. Spectrophotometers are used in waste water analysis, life sciences, enzyme kinetic studies, and industrial laboratory applications as well as for many other analyses.