The PASCO CI-6747 Colorimeter measures the amount of light that is transmitted through a liquid. The intensity of the light passing through the liquid can often be used to determine properties of the liquid such as the concentration of chemicals in the liquid. Substances absorb different amounts of certain colors of light and transmit other colors. Some substances absorb red but not blue, or green but not orange. The Colorimeter can shine the following colors of light through a liquid: orange (630 nm), green (565 nm), blue (460 nm), and red (697 nm). The Colorimeter uses a small container called a cuvette that holds a small amount of liquid. When you want to measure how much colored light can pass through the liquid, put some of the liquid in a cuvette and place it inside the Colorimeter.