Absorbance is crucial in determining concentration of a substance in a sample through colorimeter analysis. Colorimeters measure the intensity of colour and light transmittance by the sample to achieve the concentration.
A colorimeter works by shining a white light through an optical filter into a cuvette that holds the sample. By adding a colour reagent to the sample, the substance will darken in colour depending on the amount of concentration. The darkening (intensity) of the colour is measured by how much light is absorbed by the sample. The higher the intensity, the higher the concentration.
Absorbance can be defined as a logarithmic measurement of the amount of light at a particular wavelength taken in by sample. This measurement is known as the Beer-Lambert Law or Beer's Law. The law states that absorbance is equal to the concentration multiplied by the path length of light and the molar extinction coefficient (how strongly the solution absorbs colour).
The colour of the sample depends on the transmittance of light, not absorption. For example, a sample is seen as red because it absorbs blue and purple wavelengths. Therefore in that example, the wavelength of light used to determine absorbance will be in the blue region of the visible colour spectrum.