04Lab_Neon-Lights

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Astronomy

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Oct 30, 2023

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M ONTGOMERY C OLLEGE – R OCKVILLE A STRONOMY 101 ASTR101 Laboratory 4 – Neon Lights Name: Aditya Rana Spectroscopy is one of the most important tools an astronomer has. Spectra can tell us about a star’s temperature, composition, and motion. In this lab you are going to explore how spectra are formed by studying emission line spectra.. PART A Go to http://phet.colorado.edu/en/simulation/discharge-lamps Click the “Download” button and open the simulation. There are two tabs at the top of the simulation: One Atom and Multiple Atoms . For now, stay on the One Atom tab. The figure shown in the sim is a glass discharge lamp. It’s basically a glass container filled with a particular gas (hydrogen, sodium, or mercury, for example) connected to a battery. In this case there is one little hydrogen atom (labeled with the number “1” inside the glass container. When the battery is on, electrons flow into the glass container and can bump into the hydrogen atom. Let’s observe what happens when an electron and hydrogen atom collide. The higher the voltage of the battery, the more energetic the electrons will be. Let’s start our electrons out at a low energy. Slide the battery switch to 3.00 V. Select Single for Electron Production , and then fire the electron. What happens to the electron? Electron gets charged, however nothing happens at collision. Let’s see what happens when there are more electrons. Select Continuous for Electron Production and move the slider up as much as you want. (If the electrons aren’t firing, press the play (  ) button at the bottom of the screen.) What happens Same effect as single electron fire. However, there were multiple 1

ASTR101 L ABORATORY 4 to the electrons now? electrons this time. What do you think you need to do get the electrons to interact with the hydrogen atom? Increase the voltage. Stop the electrons from firing with the pause (  ) button on the bottom of the sim. Turn the voltage on the battery to about 23.00 V, and then resume the electrons firing with the play (  ) button. The electrons should now interact with the hydrogen. The Legend in the upper right-hand corner of the screen can be used to identify what is coming off the collision of the electrons with the hydrogen atom. What comes out of the collision? (Select Run in slow motion under Options if you need to.) A photon shoots out. Now look at the energy level diagram of hydrogen shown on the right (under the Atom Type ). The diagram is showing in real time how the electrons are interacting with the hydrogen atom. When an electron jumps from a higher to a lower energy level, a photon is emitted, shown by a squiggly black line. We want to look more closely at these photons. Select Spectrometer under options. A spectrum (a plot of intensity vs. wavelength) should appear below the discharge tube. The photons are being separated by energy. Uncheck the Run in slow motion box and watch the spectrum. Each time a purple photon is emitted, the spectrum records one purple square. (For example, if there are 10 purple photons emitted, then the purple line will be 10 squares high. The spectrum is essentially a histogram of the emitted photons sorted by wavelength. Let the electrons run for a few minutes. You should see several lines beginning to form. Pause the electrons and reset the spectrometer. Slide the battery to 30.00 V and the Electron Production to 100% . Fire the electrons continuously for at least two full minutes. What color lines do you see building in the spectrum? Grey, purple, blue, cyan and red 2

ASTR101 L ABORATORY 4 What color appears around 410 nm purple If you run the simulation long enough, you will see four lines appear in the visible portion of the spectrum (between 400 nm and 700 nm. Remember, a ‘nm’ is a nanometer, or 10 -9 m. It’s really small!) What you are seeing is called an emission line spectrum - bright lines on a dark background. The lines in the spectrum appear when the electron drops from a higher energy level (also referred to as an excited state) to a lower energy level. When the electron drops, it emits a photon of a specific energy. The energy of the photon is the difference between the energy levels. (The lowest energy level is referred to as the ground state.) The four visible lines you see in hydrogen result from the electron dropping from a higher energy level to the second energy level (level 2).These lines are part of what is named the Balmer series . Record the wavelengths you observe from the sim (in nm) for the four visible emission lines in the Table 1 below under Observed wavelength . Using the internet (or physics book if you have one!), look up the wavelengths of the visible Balmer lines and record in Table 1 under Actual wavelength . They should be comparable. If they aren’t, look again at the values you observed in the sim spectrometer. (Google “Balmer Series”. The Wikipedia article will have a table with the lines.) Table 1 Color Observed wavelength (nm) Actual wavelength (nm) Violet 410 410.17 Blue 435 434.04 Aqua 485 486.13 Red 655 656.279 Which visible line has the highest energy? red Which visible line has the lowest energy? violet 3

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ASTR101 L ABORATORY 4 Will a hydrogen atom emit light if the atom’s electron is in the ground state? Explain your reasoning. It will not emit light, because to emit light an electron must move from a higher energy level to a low energy level. The figure below shows the energy level diagram for a hydrogen atom. (The ‘eV’ stands for a unit of energy called an electron volt.) The dots represent electrons, and the arrows (labeled A-F) represent the electron’s “dropping” to a lower energy level. Match visible spectral lines you observed in the simulation with the transition in the Figure. The first one has been completed for you. (Remember that the visible lines are part of the Balmer series that end on level 2.) 4

ASTR101 L ABORATORY 4 Violet B Blue C Aqua D Red E PART B Select the Multiple Atoms tab. You should now see many hydrogen atoms in glass container. Make sure the Spectrometer option is selected. Set the battery voltage to 30.00 V and continuous electron production at 100%. The hydrogen emission lines should show up very quickly! Now, what happens if we put a different element into our gas discharge tube? Pause the simulation. From the Atom Type menu, select Neon . Run continuous electron production at 100% for at least thirty seconds. What colors are the emission lines you see? Green, yellow, orange, red. You’ve probably seen neon lights on restaurant or store fronts. A true neon light will contain a gas tube of neon atoms. When we look at a neon light, our eyes blend together all the emission lines (our eyes, unfortunately, do not have spectrometers installed!). The color we see is the combination of the emission lines. What color do you think a neon light would appear to your eye? Red Keep the simulation running and select “Sodium” from the “Atom Type” menu. What color do you think a sodium light would appear to your eye? yellow 5

ASTR101 L ABORATORY 4 Keep the simulation running and select “Mercury” from the “Atom Type” menu. What color do you think a mercury light would appear to your eye? (Hint: which visible line grows the fastest?) white Click on the “View Picture of Discharge Lamps” to test your predictions for the colors of neon, sodium and mercury lights. Were your predictions correct? yes Sodium and mercury are commonly used for street lights. Next time you are out driving at night, look at the colors of the lights. You may be able to determine the type of light it is by the color! PART C Please respond to the following in your own words. How is an emission line spectrum formed? It is created when electrons return from a higher energy state to a lower energy state, hence producing photons. Use your knowledge of atoms to explain why are different gas discharge 6

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ASTR101 L ABORATORY 4 lamps are different colors. Different gases produce different color discharge lamps because each element has different energy levels in its electrons. Relate what is seen in a spectrometer to what is physically happening. A spectrometer records what wavelengths are released upon a charged electron hitting an atom. 7