Using Spectroscopy to Evaluate the Absorption of Light for Different Substances

Authors Avatar

Using Spectroscopy to Evaluate the Absorption of Light for Different Substances


Laura and Polina, Section 13

BIOL 130, Monday 7:00-9:50pm Rm B2 151

Performed on October 20th, 2008


        The purpose of experiment one was to determine the concentration of the unknown solution by using spectroscopy and comparing that substance to substances of known concentrations. In experiment two, the purpose was to discover the regions of the visible light spectrum are absorbed by both samples of chlorophyll. The spectrophotometer was an exceptionally useful tool for this lab. Spectroscopy is valuable for identifying substances through absorption of light which is done by measuring substances and comparing them to other known substances. Specifically a spectrophotometer is a device that measures the absorption of radiation at a particular wavelength. This is done by a light bulb shining, refracting its light into one beam, which then passes through an exit slit, then through the test solution and to a detector. This detects the amount of light that passed through the substance and a readout shows the amount of light that was absorbed (Jones, A, et al, 2007).

        Beer’s Law states that concentration of a substance is directly proportional to its amount of light absorption (Department of Biology, 2008). The number of molecules of the solute is related exponentially to the amount of light that is absorbed while passing through the solute, called the solute concentration. Using Beer’s Law with known absorptivity at the absorption maximum a substance’s concentration within a solute can be measured. Since spectrophotometers are constructed to give absorbance, concentration can be figured out through their relationship to each other (Jones, A, et al, 2007).

        Considering the fact that distilled water was used as a blank and also used to dilute the solutions, the output of the absorbance of the different solutions is compared to distilled water. The concentration of distilled water should be zero, meaning the absorbance of light should be zero as well (Jones, A, et al, 2007).

        Experiment one used the substance fast green which is a dye. In experiment number two the substance used was chloroplast pigment (Department of Biology, 2008).

Join now!

        Chloroplasts are organelles located in mesophyll cells of leaves. They have DNA and are self replicating. These organelles are where the process of photosynthesis takes place, making plants semi self sufficient. In these chloroplasts are chlorophylls which are molecules or pigments which capture the light from the sun. Chlorophylls only absorb certain wavelengths of light and reflect certain wavelengths. The wavelengths they reflect are green in colour which is why leaves are green (Karp, 2008).

Materials and Methods

        All steps were carried out as stated in the lab manual. The only deviation that occurred was that when the absorbance reached ...

This is a preview of the whole essay