Gas Tube Spectroscopy

Transcription

1 Gas Tube Spectroscopy - 1 of 12 Gas Tube Spectroscopy Brief Summary This is an exploration station where visitors can view glowing gases through diffraction grating glasses and see the gases' spectral fingerprints. This same technique is used to identify the chemical components of stars. Very often, a substance can be energized and made to glow. Though solids at the same temperature glow with the same color (lava, red hot pokers, etc.), low pressure gases glow with their own distinctive colors. To the naked eye, it may be hard to tell one color from another but passing the colored light through a diffraction grating breaks it up into its component colors - or spectrum. Equipment Required (All Stored on Cart) Spectrum glasses Optional: Spectroscopy Cart with gas tubes pre-loaded in both spectrum power supplies Spectrum charts Bohr atom charts Colored filters Easel Tripod Main Teaching Points When energized, every kind of gas glows with a unique pattern of colored lines (called its "spectrum") which can be used to identify it. A similar technique is used to identify the composition of elements in stars and the atmospheres of planets. Our eyes blend the various spectral lines together to give us one color impression such as cherry red. When analyzed through a diffraction grating, two identical-looking color impressions may turn out to have very different spectra. 1

2 Gas Tube Spectroscopy - 2 of 12 Enriching Teaching Points The Sun s spectrum is not a result of emission lines in the way the gasses in the gas tubes are. Instead, it looks more like a continuous spectrum (rainbow). The light from the Sun is close to a blackbody, with a temperature of 5,800 K. The Sun s spectrum also includes several dark lines, called Fraunhoffer or absorption lines, which can help us determine the elements in the atmosphere of the Sun. Educational Strategy This is an on-going exploration in which you coach visitors through a series of discoveries, not a demonstration. Under the guidance of a Museum Galaxy Guide, visitors can follow their own curiosity and try several different experiments. The attractive nature of the glowing gases appeals to a variety of people so this activity can be appreciated by different people in their own way (scientifically, aesthetically) and at their own level. Set Up 1 Spectrum Tube Power Supply (for Hydrogen) 2 White Light 3 Carousel Spectrum Tube Power Supply The view of the front (guest facing side) of the cart 2

3 Gas Tube Spectroscopy - 3 of 12 On/Off Switch for Spectrum Tube Power Supply On/Off Switch for White Light On/Off Switch for Carousel Spectrum Tube Power Supply 1. Position Gas Tube Spectroscopy cart and plug in power cord. Suggested Locations: o Next to the Seeing the Sun kiosk o In the area next to the ramp where Cratering used to be located o Close to the Infrared Table 2. Test all of the Spectrum Tubes to make sure they are operating and do not need to be replaced. Note: occasionally a tube may not be connecting well, so always try wiggling them back and forth and making sure they are seated all the way down before declaring the tube dead. Test the Spectrum Power Supply, which holds hydrogen, by turning the power switch to on. Test the tubes in the Carousel Spectrum Tube Power Supply ( Carousel ). It holds all of the available spectrum tubes gasses except hydrogen. The Spectrum Tube gasses in Carousel are: o Helium o Nitrogen o Argon o Neon o Carbon Dioxide o Air The Carousel is designed to hold many Spectrum Tubes but to only power one at a time. This allows for quick switching between gasses for comparison. The slot with the power that will excite the gas and cause it to light up is at the very front of the Carousel, facing guests. 3

4 Gas Tube Spectroscopy - 4 of 12 To rotate which spectrum tube is facing forward and can be powered on, grab the black dividers located in between the spectrum tubes and move them around the Carousel either right or left (see picture below) 3. Hand out spectroscopy glasses to guests and begin facilitating. Suggested ways of presenting activity Try this: Let visitors explore the different spectra from various gas tubes using the spectrum glasses. Have visitors describe the spectra. How do the spectra of various gases differ? What may cause those differences? Or try this: Have visitors try to identify which gas is in the glowing tube by matching it with the periodic table spectrum poster. Or this: Energize two gas tubes side by side at the same time. Point out how difficult it would be to decipher which TWO substances are glowing. Now show a spectrum chart of an actual star to demonstrate its complexity. 4

5 Operating Tips Gas Tube Spectroscopy - 5 of 12 Turn off tubes when not in use since they have very short life expectancies. Gas tubes get hot this is why there is a Plexiglas shield at the front of the cart. Guests and Museum Galaxy Guides should not touch the gas tubes. Touching the tubes directly also transfers your hand oils on the tube, which shortens its lifespan. DO NOT remove gas tubes from the power supplies or rearrange their order. If a tube is burned out, please write a note on the whiteboard and a member of the Space Odyssey Operations Team will replace the tube. Questions and Answers What causes the gases to glow? Adapted from Crash Course Astronomy: Light When you turn on the Spectrum Power Supply, you are heating up the gasses in the Spectrum Tubes. When matter is heated, it gains energy. However, matter doesn t naturally want to hold on to any extra energy and so when energy is added, it wants to get rid of it. One way to get rid of this energy is to emit it in the form of light. The wavelength, or color, of light it emits will depend on temperature. The hotter something is, the more energy it has, and thus it will emit a shorter wavelength of light (astronomers refer to this as bluer light). The cooler something is, the longer the wavelength or redder light it will emit. This extends beyond the visible spectrum (the light we can observe with our eyes). In fact, humans emit light but it is in the infrared part of the spectrum, which our eyes cannot detect (see the Infrared Experiments Table for more information). Image Courtesy NASA 5

6 Gas Tube Spectroscopy - 6 of 12 The emission of light from the gasses in the Spectrum Tubes, and their specific colors, is occurring because of what is happening at the atomic level. Atoms are the building blocks of all matter. They are made of three subatomic particles: protons, neutrons and electrons. Protons have a positive charge, neutrons no charge, and electrons have a negative charge. The protons and neutrons are heavier and are located at the center of the atom, called the nucleus. The negatively charged electrons are attracted to the protons and whiz around the nucleus. Often times you will hear the analogy that electrons orbit the nucleus of the atom like planets orbit the Sun. This is a simplified explanation which is a good mental model; however, the reality is much more complicated and requires quantum mechanics to describe. The important thing to know is that the electrons are only allowed to occupy certain volumes of space and what volume they occupy depends on the energy of the electron. One way to visualize this is to think of the atom like a staircase. The nucleus is at the landing and the electrons occupy different stairs, or energy levels. The higher on the staircase the electron is, the more the energy it has. The electrons need energy to go up the stairs and they can only go up by whole steps each time (just like you walking up the stairs, they cannot be on stair 9 ¾, for example). The electrons need a very specific amount of energy to be able to go up to the next step. If they don t have enough energy, Image from Crash Course Astronomy: Light they cannot move up to the next step, or energy level. Electrons can move up and down the stair case one step at a time, or they can jump more than one step if they have the right amount of energy. Electrons can also move down the staircase. As they jump down steps, they give off a certain amount of energy that is equal to the amount of energy it took to jump up a step. In astronomy, electrons gain energy from heat or light. Electrons can absorb light at a very certain energy level which allows them to jump up to another energy level, or step. Remember, light is energy. So, these different amounts of energy correspond to different wavelengths. In the visible part of the spectrum, you could say they correspond to different colors. Electrons can also jump down a step and emit light at a specific amount of energy. These different specific wavelengths, or colors, are what we see as an element s spectra. Demo: show visitors the Periodic Table of Spectra Poster. 6

7 Gas Tube Spectroscopy - 7 of 12 Why are there lots of colored lines in a substance's spectrum? Each element only allows electrons to be at very specific energy levels (sometimes referred to as orbits and what were the steps in our staircase analogy). However, an electron can jump down from one energy level to another that is several steps lower in a number of ways. The diagram below shows a simplified atom. Each circle is a different energy level. Suppose that the electron started at the lowest energy level or "Ground State" (inner-most circle) and was energized with the right amount of energy to jump one of the higher energy states. When it falls back to a lower state, it gives off a photon. The electron could fall down to any of the lower states and doesn t have to go back to the energy level it started at. Each possible "drop, or transition, corresponds to a differently colored photon, depending upon how much energy was lost in the drop. A small drop in energy produces a lower energy photon (red), while a large drop produces a higher energy photon (purple). When you energize an actual gas, there are an enormous number of individual atoms or molecules that you are energizing. For some reason, certain transitions are more likely to occur than others. The more often a particular transition occurs, the more photons of that color are given off, and the brighter that colored line is in the spectrum. In summary: the color (and position or wavelength) of the lines in the substance's spectrum corresponds to all of the possible ways that its electrons can drop. The brightness of the lines corresponds to how often that particular transition occurs. Demo: Show visitors the Spectral Lines chart. Ask which spectral lines correspond to which transitions. The graph on the bottom of the chart is a more accurate way of presenting the spectrum. The height of the peaks represents the brightness of the colored lines. Why do different gases glow with different colors? Each atom, or molecule, has its own arrangement of protons, neutrons, and electrons. As a result, each has a different set of allowable energy levels that electrons might occupy, and thus a different set of colors in its spectrum. Going back to our staircase analogy, each atom has a different staircase and the steps may be at different heights and require more or less energy for an electron to jump up or down to the next energy level (step). Compare the image to the right which 7 Image from Crash Course Astronomy: Light

8 8 Gas Tube Spectroscopy - 8 of 12 shows the staircase for helium to the staircase for hydrogen shown in the first Q&A question. Your eye and brain combine all of the colors in a substance's spectrum into a single color. However, when you look at that light with a diffraction grating (which are in the glasses we provide at the cart) or a spectrometer, you will see a different combination of distinct color lines. These distinct lines make up an element s unique light fingerprint. It is because of this property of atoms that we can look at nebulas, stars, and planets light years away and tell what they are made up of. Demo: Suggest that visitors experiment with the Spectroscopy interactive where they are able to mix colors to see how their eyes/brain perceives various mixtures. Why do some spectra have only a few lines and others have whole bands of color? In the illustrations above, we have shown only one electron and its various possible energy levels. This resulted in a spectrum with only a few lines. Now imagine an atom with many electrons. There will be more possible energy levels, and more electrons jumping from energy level to energy level. This will cause many more spectral lines. Taking this a step further, when atoms combine to form molecules possible energy levels arise not only from each of the atoms, but also from the interactions between the atoms. The result is often that there are so many lines in the spectrum that they almost seem to blend into a continuous band. Compound this with the fact that a molecule can vibrate and rotate which can also produce photons, and you have a spectrum that can be quite complicated. As a rule of thumb, simple atoms (like Hydrogen or Helium) have simpler spectra than elements higher on the periodic table and molecules have more even more complicated spectra. Demo: Show visitors glowing gas tubes and have them guess which are produced by glowing molecules and which by glowing atoms. Isn't it true that everything you've said so far applies to non-visible light as well as visible light? I mean, doesn't a glowing atom or molecule give off light in the infra-red region, or the ultra-violet region? And isn't it true that scientists study the spectra of astronomical objects in many different wavelengths, like x-rays, or microwaves, not just the visible-light wavelengths? Yes. Demo: Suggest that visitors check out the Infrared Experiments interactive to experiment with infra-red light. They should also check out the Sun in Many Wavelengths interactive to see other types of light that the Sun gives off in addition to the visible light we are familiar with. Why does the Sun s spectrum look more like a rainbow rather than individual spectral lines? (Adapted from The Sun s spectrum is close to a blackbody, with a temperature of 5,800 K. A blackbody is an idealized object that absorbs all of the light it receives (meaning none of it is reflected

9 Intensity (brightness) Gas Tube Spectroscopy - 9 of 12 away or passes through it and out the other side). The energy that the object absorbs causes it to heat up. Remember, objects don t naturally want to hold on to extra energy. So, these objects get rid of the extra energy by emitting light, but they do so in a continuous spectra (all wavelengths of light). There are a couple of important properties of the light perfect blackbody objects emit: 1. The hotter the blackbody, the more light it gives off at all wavelengths. That is, if you were to compare two blackbodies, regardless of what wavelength of light you observe, the hotter blackbody will give off more light than the cooler one. 2. The spectrum of a blackbody is continuous (it gives off some light at all wavelengths), and it has a peak at a specific wavelength. The peak of the blackbody curve in a spectrum moves to shorter wavelengths for hotter objects. If you think in terms of visible light, the hotter the blackbody, the bluer the wavelength of its peak emission. For example, the Sun has a temperature of approximately 5800 Kelvin. A blackbody with this temperature has its peak at approximately 500 nanometers, which is the wavelength of the color yellow. A blackbody that is twice as hot as the Sun (about K) would have the peak of its spectrum occur at about 250 nanometers, which is in the UV part of the spectrum. (Temperature in Kelvin) Wavelength (color) There is no object that is an ideal blackbody, but many objects (stars included) behave approximately like blackbodies. Other common examples of objects that give off blackbody radiation are the filament in an incandescent light bulb or the burner element on an electric stove. As you increase the setting on the stove from low to high, you can observe it produce blackbody radiation; the element will go from nearly black to glowing red hot. 9

10 Gas Tube Spectroscopy - 10 of 12 This concept explains why there are different colors of stars. Red stars are cooler, and they emit the most radiation in the red wavelengths. A hotter star, like the Sun, emits the most radiation in the yellow/green part of the spectrum. Hotter stars, like Sirius, appear blue. But the Sun s spectrum isn t a perfect rainbow. What are those dark lines? As we mentioned before, the Sun is not a perfect blackbody. The temperature dependent light that it emits passes through the outer atmosphere of the Sun, where it interacts with the atoms in the gasses of the atmosphere. Some of that light is at exactly the right energy level for electrons in those gasses to jump up a step (orbital). When this happens, we see what astronomer s call absorption lines or Fraunhoffer Lines, named after the scientist who discovered the dark lines in the solar spectrum. Just like the spectra we had discussed before, which is called Emission Spectra, these lines correspond to specific elements. Scientists can identify each of these lines which tells them the composition of the solar atmosphere. For instance, the two lines close together in yellow (marked D on our Solar Spectrum poster) come from sodium. How do we observe spectra? Scientists use spectrometers or diffraction gratings to be able to observe the spectra of individual elements as well as spectra from stars, planets, nebula, and galaxies. Similar to a prism, both of these instruments split light into its component colors, or wavelengths. The spectroscopy glasses on the cart use diffraction gratings. A diffraction grating is a material that has several parallel, closely placed ridges on it. Diffraction gratings have an advantage over prisms in that they are not as sensitive to color, and thus can be used outside of the visible part of the spectrum and they produce a stronger (brighter) spectra. This is especially important for astronomy, where we are often looking at very faint objects. Diffraction grating produce more than one image of the spectra, which you might notice when looking through the spectroscopy glasses. 10

11 Gas Tube Spectroscopy - 11 of 12 What is the layout of the gas tubes in the Carousel Spectrum Power Supply? 11

12 Gas Tube Spectroscopy - 12 of 12 Other Cool Stuff to Try Use the spectrum glasses to look at other light sources around the room. With your spectrum glasses on, look at glowing gas tubes through colored filters. Notice how the filters don't add any color; they actually subtract parts of the spectrum! For example, a green filter subtracts all but the green light. (The graphs that go with each filter show which colors will pass through and which will be blocked.) Gases that surround stars can also subtract parts of the star's spectrum. This results in black lines, called Fraunhoffer Lines, which are sometimes also referred to as absorption lines. Fast Facts In 1666, Isaac Newton was the first to discover that sunlight could be broken into a rainbow of colors, then recombined into its original white color. Because of the magical way the rainbow appeared on his wall, he coined the term "spectrum" which is the Latin word meaning apparition or ghost. Joseph von Fraunhofer discovered the dark lines in the Sun's spectrum in 1814 and realized they were gaps of missing colors. He also invented the diffraction grating which is a much more efficient than using a prism to create spectra. The science of spectroscopy was created in 1859 by Gustav Kirchoff and Robert Bunsen. They invented a device that used a prism to break up the light of substances being burned in a Bunsen Burner. Potential Problems Gas tubes can break. Be careful during handling of tubes and transportation of cart. If a gas tube breaks do not try to clean up the broken glass yourself. Contact Security Subpost (x6343 from the phone in the 2003 room) and they will send someone to clean it up. In the meantime, please stanchion off any area on the floor where there is broken glass and make sure guests to not walk on it or pick any pieces up. Background materials (websites, videos, articles, digital collections links) Periodic Table of the Elements with Spectra: Spectroscopy 101: The Solar Spectrum: Crash Course Astronomy, Light: A timeline of atomic spectroscopy: &date=&pageid=1 MIT George R. Harrison Spectroscopy Laboratory website: 12