Spectrophotometers are used to measure the absorbance or transmittance of a solution. The spectrophotometer measures the amount of light transmitted through a sample. A cuvette is a small container that holds a liquid sample. The cuvette has two sides, one side is coated with an absorbing compound called a reagent. When the cuvette is placed into the spectrophotometer, it absorbs some of the light passing through the sample. This causes a change in color of the reagent which can be measured by the instrument. The wavelength of light absorbed depends on the chemical composition of the sample. By measuring the absorption of light at different wavelengths, the concentration of the sample can be determined.