Spectrometry Lab

Caleb W. Skocy

AST 115 Honors

9 April 2016

Spectrometry Lab

Introduction

            In this lab, we familiarized ourselves with a method used by astronomers in identifying the composition of astronomical objects.  We used spectrometers to observe the emission spectra given off by several different elements.  The spectrometer diffracts the light and reflects the colors onto a scale.  This scale measures the wavelengths of the light.  Using this emission spectrum, we can identify the composition of the light source.  This is called spectroscopy. 

Procedures

Part 1: Observing Spectra Using a Diffraction Grating

A.    Your instructor will provide you with a spectrometer with which to observe spectra. The most important part of the spectrometer is called a diffraction grating. The grating in your spectrometer is used to disperse the light into a spectrum for viewing on the spectrometer scale. Before using the spectrometer, study the 2”x2” diffraction grating found in your lab packet. This sample grating is similar to the one in the spectrometer. We noticed bright colors seen through the grating and also reflected off its surface. A grating uses the wave phenomena of diffraction and interference to produce spectra. A prism, on the other hand, uses the fact that different colors of light are refracted at different angles as they pass from air through glass.

B.     Holding the grating close to one eye and looking through it toward an incandescent light bulb, you should see a continuous spectrum of color off to either side of the bulb. You may have to shift your head around somewhat to see these. Describe what you see, and pay careful attention to the order of the colors and locations compared to the light bulb.

C.     Using the spectrometer: The remainder of the exercise will be done using the spectrometer. One looks through the diffraction grating at the narrow end of the device. Notice that there is a narrow opening, or “slit”, opposite the diffraction grating. Hold the end with the diffraction grating near your eye. As you look through the grating, find the slit. Holding the spectrometer steady near your eye, twist your upper body slowly horizontally, left and right, until you see light from a source. Then, without moving your body, glance to the right of the interior of the spectrometer and see the spectrum displayed on the scale. Use the wavelength scale that is labeled 700 through 400 running from left to right. These are wavelength values in units of nanometers (where 1nm = 10^-9m). Note that the corresponding range in units of Angstroms would be from 7000 through 4000. You may need to calibrate the wavelength scale of your spectrometer. Your instructor will explain how. If you have any difficulties using the spectrometer, be sure to ask your instructor for help.

Part 2: Observing the Continuous Spectrum of an Incandescent Light Bulb

A.    You now need to look at the spectrum of a light bulb through the spectrometer. You should see all the colors of the spectrum aligned with the wavelength scale. When you use your calibrated spectrometer to observe the spectrum, notice that each color falls on a different portion of the wavelength scale. Describe what you see, and what colors correspond to what wavelengths.

B.     Look carefully at the left edge of the spectrum and note the wavelength scale reading at which the light disappears. Do the same on the right edge of the spectrum. These are the wavelengths “limits” of the sensitivity of your eye. Record these wavelength numbers.

Part 3: Observing the Emission Spectra of Several Elements

A.    In this section, you will need to look at several spectrum tubes, each filled with the gas of a specific chemical element. When a high voltage is applied to the ends of the spectrum tube, current flows through the gas and heats it. A hot gas emits only certain colors of light. Each type of gas produces a unique pattern of bright spectral lines. As you view each tube with your spectrometer, note the wavelengths of dominant bright lines of each element. Describe what you see. Also note the general color of the tube of gas as seen with the un-aided eye in your blog post. Hot gasses do not produce a continuous band of colors, and the unaided eye will see only one combined color, which will be a mix of all the different colors in the spectrum of a particular element. Use this information to identify the unlabeled gasses. You should study the emission spectra of hydrogen, helium, mercury, neon, and sodium.

Part 4: Observing Other Sources of Light

A.    Look at a florescent light bulb with your spectrometer. Note that it appears to have a continuous spectrum with emission lines on top of it. Record what wavelengths you observe the emission lines to be at.

B.     Now observe two other light sources of your own choosing (either in the building or outside). Note: During the day, NEVER point the spectrometer directly at your sun. You could damage your eyesight. It is safe, however, to observe the solar spectrum reflected off white clouds, concrete surfaces, or the moon.

C.     Compare these spectra to your previous results. Try to determine which gasses are in these lights by the kind of spectra they have and by the patterns of the spectral lines that they produce.

Results and Discussion

Part 1: Observing Spectra Using a Diffraction Grating

B.      Spectrum of an incandescent light bulb as shown by the sample diffraction grating.

1.      Incandescent Lightbulb

As we can see in the picture above, the diffraction grating shows the incandescent light with a continuous spectrum, from the shorter wavelengths starting with violet on the left, to the longer wavelengths ending in red of the right.

Part 2: Observing the Continuous Spectrum of an Incandescent Light Bulb

A.    The spectrometer shows a continuous spectrum all the way from wavelengths of 700nm to 400nm.  From 710nm-625nm, the light appears red.  From 625nm to 600nm, the light is orange.  Then, from 600nm to 570nm, the color is yellow.  From 570nm to 500nm, the light is mostly green.  From 500nm to about 435nm, the light is blue.  Then the light starts to turn violet, from 435nm to 400nm. 

*This spectrum is also recorded in Table 1.

B.     The spectrum ends at 710nm and 400nm.  This is the range of visible light that can be seen by the unaided human eye.  After the red light at 710nm the light becomes infrared.  Before the violet light at 400nm, the light becomes ultraviolet.

Part 3: Observing the Emission Spectra of Several Elements

·         All the data and pictures for the spectrums of the tubes of gas observed are recorded in Table 1.

Table 1: Appearance and Emission Spectra of Observed Gases

Station #	Appearance	Emission Lines (nm)	Spectrum Description	Element
1	White	Continuous spectrum from 710 to 400	Continuous Spectrum in all visible wavelengths	Incandescent Lightbulb
2	Bluish-Purple	Red-660 Teal-490	Very intense at 660nm and 490nm, with many low-intensity emission lines in between and before 490nm	Hydrogen
3	Reddish-Orange	Red-705, 695, 675, 670, 661, 656, 654, 636, 634, 630 Orange-625, 620, 610, 605 Yellow-600, 592, 589 Lime-580 Green-542, 539	Numerous intense emission lines	Neon
4	Bluish-White	Yellow-590 Lime-560 Green-515 Blue-470 Violet-430	Large number of low-intensity emission lines, making the spectra appear almost semi-continuous	Krypton
5	Pinkish-Peach	Red-710, 670 Yellow-590 Teal-505, 495 Blue-475, 450	Intense emission lines, extremely intense at 590 (yellow)	Helium
6	Yellow	Orange-625 Yellow-590 Lime-570 Green-515 Teal-500 Blue-470	Very intense emission lines, especially intense at 590 (yellow)	Sodium
7	White with Greenish tinge	Yellow-580 Green-550 Teal-495 Blue-440 Violet-400	Intense emission lines, most intense at 550 (green) and 440 (blue)	Mercury

Part 4: Observing Other Sources of Light

A.    The emission lines over the continuous spectrum of the fluorescent light were very intense at 615nm (red), 550nm-543nm (green), and 440nm (blue).  There were also some fairly strong emissions at 590nm (yellow) and 490nm (teal).

B.     We observed the reflection of the incandescent light bulb off of my purple notebook and translucent red clipboard.

                                   

1.      Purple Notebook              2.   Red Clipboard

C.    Originally, the incandescent lightbulb gave off a continuous spectrum of light, from 710nm to 400nm.  With the purple notebook, the all wavelengths longer than 470nm (blue) seemed to disappear, while the remaining purple and blue light also became less intense.  With the red clipboard, the only light remaining were all wavelengths longer than 600nm (orange). The remaining red and orange light was much less intense.  We got these results because of the color of the objects. The purple notebook would only be reflecting the blue and violet wavelengths and the clipboard would be reflecting red wavelengths, this is why they both appear the colors they are.

Conclusion

            In this lab, we learned much about the methods of spectroscopy.  We learned how to use spectrometers to measures the wavelengths of lights given off by hot gases.  We learned how to identify elements according to spectra, and identified the elements observed in class by using their spectra.  Overall, we learned how useful the method of spectroscopy can be to astronomers in identifying the composition and conditions of astronomical objects in the Universe.