Spectroscopy Lab - Gates Bartz

Purpose:

In this laboratory exercise, we had an opportunity to become familiar with the methods used to determine the basic chemical composition of astronomical objects. We learned about diffraction and observed the spectral characteristics of several chemical elements and other materials and learned how to identify them by the spectral lines they emit or absorb.

Introduction:

Spectrum = an array of colors or other electromagnetic radiation produced by a prism or similar device (Plural: Spectra). The study of spectra is one of the most important parts of observational astronomy. Analysis of the lines appearing in an objects spectrum permits the astronomer to understand the conditions present on the object emitting the light. Careful determination of the positions, strengths, and shapes of these lines can help to determine an objects surface chemical composition, motion, surface temperature, rotation, speed, rough size, and other properties. Much of all that we have discovered about objects in the Universe has been learned from studying their spectra.

Results:

Part 1: Observing Spectra Using a Diffraction Grating

1.              Our instructor provided us with spectrometers 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 is used to disperse the light into a spectrum for viewing on the spectrometer scale. Before using the spectrometer, we studied the 2”x2” diffraction grating found in our 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.

2.              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.

·       The continuous spectrum shown through the sample grating is a rainbow of color from red to violet. With the sample grating, several of these spectra appear around the light seemingly “blooming” from it.

3.              Using the spectrometer: The remainder of the exercise will be done using the spectrometer. Look 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. (Most are already calibrated, mine was.) 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

1.              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.

·       The colors of the continuous spectrum shift from (left to right) red to violet. Red is most present around 700-625nm. From 625-600nm the light is more orange. 600nm to about 575nm is yellow. Between 575-500nm is green. 500-475nm shows a light blue, while 475-425 is a darker blue. And last but not least, from 425-410nm (where visible light ends) the color is a deep violet.

2.              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.

·       The light we see is what is visible, beyond red is infrared and beyond violet is ultraviolet. The wavelength of visible light that I can measure with the spectrometer starts with the longest visible wavelength of 700nm which appears red. The shortest wavelength of visible light that I can measure with the spectrometer is 410nm which appears violet.

Part 3: Observing the Emission Spectra of Several Elements

1.              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.

Color of Gas to the Naked Eye	Wavelength of Emission Lines	Description	Identity of Gas
White	Continuous Spectra (no emission lines)	Continuous spectra from red to violet	Incandescent Light Bulb
Light Purple	*655nm (red) 590nm (yellow) 485nm (teal)	Many low-intensity emission lines throughout spectra	Hydrogen
Red-Orange	640nm (red) 615nm 610nm (orange) 595nm 589nm 586nm (yellow) 530nm 540nm (green)	Many high-intensity lines at the red/yellow end of the spectra	Neon
Light Blue	*587nm (orange) *557nm 515nm (green) 470nm (blue) 430nm (purple)	Appears to have a blur of low-intensity emission lines	Krypton
Light Yellow	706nm 668nm (red) *590nm (orange) 502nm 495nm (green/teal) 472nm 450 nm (blue)	Distinct high-intensity emission lines	Helium
Yellow-Orange	616nm (red) *590nm (orange) 570nm 515nm 500nm (green) 466nm (blue)	Distinct high-intensity emission lines	Sodium
White	*570nm (orange) *547nm 590nm (green) *435nm (blue) 406nm (purple)	Distinct high-intensity emission lines	Mercury

*line that appeared the most intense in the spectra

Below is a table showing optimal examples of the emission lines from the elements above.

Part 4: Observing Other Sources of Light

1.              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.

·       612nm (red) 547nm 542nm (green) 435nm (blue)

2.              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.

·       I observed two different brightly colored objects under the incandescent light bulb.

3.              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.

·       Originally, you would see a continuous spectrum with an equal intensity of all the colors. After putting a brightly colored object underneath, there was a shift in intensity of the colors. I observed a bright purple notebook underneath the incandescent bulb, and the spectra became much duller on the red side and much more intense on the violet side. The opposite happened when I observed a bright red binder under the incandescent light bulb. The red stayed intense, and the violet end of the spectrum appeared to be dulled.

Conclusion:

Astronomers are able to determine the chemical composition of many distant objects in space by using their emission spectra. If we can observe light from the object, we can take an educated estimate of its composition. This is an incredibly valuable technique, and only one of many that use light to measure different properties of foreign bodies in space.