Spectroscopy

Background:
Spectroscopy is the study of light as a function of wavelength that has been emitted, reflected, or scattered from a solid, liquid, or gas.  FIDO uses imaging spectroscopy in the field to identify the material composition of rocks.  As photons (light particles) enter a mineral, some are reflected from grain surfaces, some pass through the grain (refract), and some are absorbed. Those photons that are reflected from grain surfaces or refracted through a grain are scattered. The amount of scattering versus absorption controls the amount of photons we receive from a surface. Photons may also originate from a surface, a process called emission. All natural surfaces emit photons. Each different type of atom or ion emit their own unique set of light waves. These light waves are also called emission lines, because light at these particular wavelengths looked like many straight lines in a spectrum. Scientists use these emission lines to identify atoms, similar to the way a detective uses fingerprints to determine whose hands have touched an object. Once scientiests have determined which atoms are present, they know immediately which elements are there (like hydrogen, helium, carbon, nitrogen, etc).

The emission and absorption spectra of sodium are compared at left. The upper diagram shows the emission lines (two bright lines in the yellow part of the spectrum around 590 nm). The bottom diagram shows the absorption spectrum. The two dark lines appear at exactly the same wavelengths as the bright lines in the emission spectrum.

Activity:
Students can use visible spectroscopy to investigate the relationship between the color of a solution and the wavelength of light that it absorbs. Visible wavelengths are just a small part of the entire electromagnetic spectrum. When white light shines on a transparent, colored solution, some of the light's wavelengths are absorbed (removed) by the solution. The remaining wavelengths pass through the solution and give the solution its color. For example, a solution that absorbs green light, transmits red and blue light which makes the solution appear purple. The transmittance of red and blue light through this solution is high, while the transmittance of green light through this solution is low.

Materials:

Procedure:

1. Connect and turn on the calculator and CBL. Start the CHEMBIO application.
2. Setup one COLORIMETER.
3. Fill the first cuvette with distilled water and cap it. This is the "blank" cuvette.
4. Fill the second cuvette with the blue solution and cap it. Fill the third cuvette with the red solution and cap it.
5. Note: It is important that the cuvette is positioned correctly in the colorimeter. Two sides of each cuvette are ribbed and two are smooth. Make sure that whenever you insert a cuvette into the colorimeter that the smooth sides are next to the white reference mark on the top right side of the colorimeter.
6. To calibrate the colorimeter, place the blank cuvette into the colorimeter and close the lid. Turn the wavelength knob to the 0%T position. Press the TRIGGER button on the CBL and enter zero on the calculator. Turn the wavelength knob to the Blue position. Press the TRIGGER button on the CBL and enter 100 on the calculator.
7. Remove the blank cuvette leaving the wavelength knob in the Blue position.
8. Choose to MONITOR INPUT from the Data Collection menu.
9. Place the blue cuvette into the colorimeter and close the lid. Wait for the calculator reading to stabilize and enter the absorbance value into the table below. Remove the blue cuvette.
10. Remove the blue cuvette. Place the red cuvette into the colorimeter and close the lid. Wait for the calculator reading to stabilize and enter the absorbance value into the table below. Remove the red cuvette.
11. Repeat steps #6-10 at the green and red wavelength knob positions.

Analysis:
The transmittance of a given wavelength of light through a solution is the ratio of the intensity of the light transmitted through the sample to the intensity of light striking the sample. The absorbance of a solution at a particular wavelength is inversely and logarithmically related to the transmittance of the solution: A = log (100 / %T). Use your data to complete the table below.

A scatter plot of the absorbance of a solution versus wavelength is called an absorption spectrum. Enter the wavelengths into list #1 on your calculator and the absorbance values for the blue and red solutions in lists #2 and #3. Construct the absorption spectrums for the blue and red solutions.

Extension:
The data in the table below were obtained for an unknown solution. Use the relationship between percent transmission and absorbance to complete the table. Based on this data, what color was the unknown solution. Explain your reasoning.

Credits:

• Astronomy Today
• Dr Jeffrey W Brosius, Raytheon STX Corporation
• Dr Ray Arvidson & staff, Washington University
• Texas Instruments
• Vernier Software

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