Spectrometry Lab

Spectrometry Lab

Abstract

In this lab, students gained skill in using a spectrometer to identify the composition of different sources of light based on their respective spectra. Students also analyzed spectra to gain understanding of how different colors of light correspond to different wavelengths. 

Introduction

When a light source is directed through a prism or grating, it is split into an array of colors representing different wavelengths of light, called a spectrum. By analyzing emission or absorption lines produced in spectra, scientists can deduce the composition and temperature of the light source, determine if the light source is moving toward or away from the observer, and gain knowledge over many other properties of the light source. In this lab, students used a grating and spectrometer to observe emission lines and spectra of several gasses or light sources. 

Procedure

Materials: Spectrometer, 2"x2" diffraction grating, an incandescent light bulb, and six stations with mystery gasses to be identified.

Sources: I found a useful website to help with identification of gasses at the six stations. This site provides a list of elemental gasses and their spectra:  http://astro.u-strasbg.fr/~koppen/discharge/

Part I:

 In part I students became familiar with a grating and spectrometer. Students learned the parts of the spectrometer, how to use it, and how using a spectrometer compares to using a grating. Students observed the ceiling light and its spectrum using both devices.

Part II:

 In part II, students use the spectrometer to observe an incandescent lightbulb. Students note and identify some features of the continuous spectrum produced, including how colors correspond to wavelengths of light and the range of wavelengths visible to the eye.

Part III:

 In part III students worked their way through the six stations of gasses set up by the instructor. Students use the spectrometer to observe the emission lines produced by the different spectra and used these features to identify the gas. To identify these, I used the website listed above to compare the spectra I observed with known spectra of elemental gasses.

Results and Discussion

Part I:

A. While (A) is not necessarily a question for students to answer or a task to perform, it provides valuable information on a diffraction grating, the use of a diffraction grating in a spectrometer, and the different ways in which spectra are produced by a glass prism or a diffraction grating. In a prism, different wavelengths of light bend at different angles as the light passes from the air through the glass. A diffraction grating, however, uses diffraction and interference of wavelengths to split the light into a spectrum. The image below illustrates the different ways that light behaves when producing spectra using a prism or a diffraction grating.

1. A grating uses diffraction and interference to produce spectra.
2. A glass prism bends different wavelengths of light at different angles.

B. When holding the grating up to the light source on the ceiling, I observed "images" of the light in different colors, which spread out symmetrically on either side of the original light. The different wavelengths of light were represented by these colors, which were violet, green, yellow-orange, and red. These colors are listed in order of appearance, with the violet being closest to the light source and red being the farthest from it.

C. This section provides students with information on spectrometers, including their parts, how to use them, and how to read the scale on which spectra are displayed within the device. 

Part II: 

A. Students used the spectrometer to observe the spectrum produced by an incandescent light bulb. The spectrum observed was continuous and contained all visible wavelengths of light. Violet/blue is observed from about 420 nm to 470 nm, blue/green is observed from about 480 nm to 540 nm, yellow/orange from about 550 nm to 630 nm, and finally, orange/red light is observed between 640 nm and 695 nm. These colors and their corresponding wavelengths in the spectra of an incandescent light bulb are shown in the image below.

Continuous spectrum produced by an incandescent light bulb

B. The lower limit of wavelengths one can detect with their eye is about 410-420 nm. The upper limit is about 690-700 nm.

Part III:

A.  Students observed spectra produced by six different light sources and used these spectra to identify the composition of the source.

1. This light source, the only one which was not an elemental gas, was an incandescent light bulb which produced a continuous spectrum similar to the one pictured below. The light produced was white.

2.  I identified this light purple gas as hydrogen. Its spectrum had three distinct emission lines at about 430nm (violet), 485 nm (blue) , and 660nm (red). Below is an image of hydrogen's spectrum obtained from the website mentioned previously.

3. This gas, which glows red when heated, is identified as neon. Neon produces many emission lines, with several in the red/orange/yellow part, between about 700 and 620nm. A green emission line is observed at about 540nm and several are observed in the blue/violet area, between 500 and 420 nm. A picture of Neon's emission spectrum is included below.

4. When heated, this gas produces a light violet/white light. Identified as krypton, this gas produces many emission lines, which occur most frequently in the blue/violet region, between 400 and 540 nm. Several lines are observed between 500 and 570 nm, and the most intense emission lines are located at 585 nm, 560 nm, and about 475 nm. 

5. Helium produces a pale yellow/white light when it is heated. Helium produces intense emission lines at 445 nm (blue), 500 nm (green), 580 nm (yellow, also the most intense), and 670 nm (red).

6.  This gas, which produces a bright yellow light, is identified as sodium. Sodium's emission spectrum produces lines at 500 nm (blue),  570 nm (green), and the most intense line at 590 nm (yellow). 

Conclusion

This lab helped students gain valuable and interesting skills in using a spectrometer and diffraction grating to analyze emission spectra and identify the composition of the light source. Additionally, students were able to practice precise measurements of wavelengths and learn the properties of emission spectra for elemental gasses. Most importantly, I believe this lab helped me to appreciate the difficulty of the work many astronomers perform everyday.