Spectroscopy

Spectroscopy – Lab 9

Read Chapters 5 &17

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Introduction

 

The data that physicists use in order to study astronomical phenomena mostly consist of electromagnetic waves.  Scientist use the entire electromagnetic spectrum in order to learn more about the natural world.  This lab will focus on the properties of light that allow scientists to analyze the composition of stars, nebula, and various astronomical phenomena.

 

  1. Spectroscopy

 

Spectroscopy is the branch of physics that is concerned with analyzing the spectrum of light that an astronomical body produces.  In physics we analyze the electromagnetic spectrum in order to obtain meaningful data.  The electromagnetic spectrum is the full range of electromagnetic radiation at different wavelengths, which vary from radio waves (which can be several meters in length) to gamma rays (which are 10-11 meters in length).  The visible spectrum of light, which is the portion of the spectrum that humans can perceive, has wavelengths from 400 – 700 nm.  A spectrum shows how much energy has been imparted to each specific wavelength of light.  This is illustrated in Figure 1 below.

Figure 1. The electromagnetic spectrum.  We will be analyzing the visible portion of the spectrum in this lab.

 

Light is composed of photons or particles of light.  When the Sun fuses hydrogen into helium, that reaction gives off photons.  When light is emitted from these processes it is emitted in a continuous spectrum.  Including some infrared and ultraviolet radiation, the entire visible spectrum is present in the light that is emitted from the Sun.  The wavelengths of light that are emitted by a star are dependent on the temperature of the star.  The temperature of the Sun is around 5,777 K.  Therefore our sun produces mostly yellow and green light.  This is illustrated in Figure 2.

 

Figure 2.  The spectrum of light that is emitted by a star depending on its temperature.  The sun is approximately 5,777 K and as can be seen from the graph produces mostly yellow green light.  However, you can see that some infrared and ultraviolet (the portion of the spectrum that gives you sunburn!) is also present.

 

Using a continuous spectrum is advantageous for producing a visible image, but if we extend our analysis to the quantum behavior of light we gain more information about the source that is emitting that light.  All atoms contain protons, or particles with a positive charge, neutrons, a particle with no net electric charge, and electrons, a particle with a negative electric charge.  All of these particles exhibit quantum behavior, and roughly speaking this means that they have discrete energy levels and positions within matter.  In particular, the Bohr model of the atom, has electrons in discrete energy levels.  The Bohr atom is shown below in Figure 3.

 

Figure 3.  The Bohr atom.  Each electron inhabits a discrete energy level within the orbit of the nucleus.

 

Electrons are particularly important within the Bohr atom.  Electrons can only inhabit specific orbits or discrete energy levels of the nucleus. The inner most orbit of the nucleus, labeled 1 in the diagram, is what is known as the ground state, or the lowest energy state within the atom. The electrons can never get closer to the nucleus than the ground state.  If a photon or some other form of energy is imparted to the electron, then it is raised to level 2 or it’s excited state.  Since electrons like to jump to different orbits depending on their amount of energy, these orbits are referred to as energy levels.

You may be asking why these energy levels are so important to physics and astronomy.  Well precisely because electrons do not like to stay in an excited state.  They must return to their ground state or their original orbit.  However, an electron cannot forget about the energy that it received to move it to its higher energy level.  Therefore, it releases that energy as a photon.  Since the energy levels between each orbit are quantized (meaning they have a specific value which does not change), the photons emitted always have the same energy.  For example, when an electron returns to it’s ground state in an element of hydrogen, it will produce a characteristic photon.  This is true of all elements, and from this fact we can analyze the light emitted from objects to determine their composition!  This is illustrated in Figure 4.

 

Figure 4. Diagram of an electron being raised to an excited state and then returning to its ground state.  A photon with a specific wavelength will be released.  From this emission we can determine the type of element present.

 

Now depending on the location and type of light source we will have different spectrums of light produced.  These are illustrated in Figure 5 below.  The first mentioned previous is a continuous spectrum.  This is a spectrum with all wavelengths of visible light present.  An example would be light produced from the fusion within the Sun.  The next type of spectrum is an absorption line spectrum, labeled C in the Figure 5. This spectrum is produced when a colder gas is in front of the originally emitted light.  It will be a continuous spectrum with dark lines superimposed over certain wavelengths.  These dark lines correspond to photons that were absorbed by the elements of the gas raising their electrons to their excited energy states.

The next type of spectra is an emission line spectrum, labeled B in Figure 5.  This type of spectrum has bright lines corresponding to different wavelengths of light.  Specific patterns of light indicate the presence of specific atoms.  These are produced from the gases in the upper atmosphere of stars, and by specific gaseous elements on Earth.

 

Figure 5. Diagram of the different types of spectra and their sources.  By combining these different types of spectra scientists can determine the composition of various astronomical bodies.

 

Figure 6.  Image of the different spectra produced by the Sun.  Scientists were able to determine the composition of the Sun by analyzing these spectrums.

 

By combining the date from the absorption and emission spectrums scientists can identify the composition of stars and nebulae.  An example of this would be the absorption and emission spectrums of the Sun, as shown in Figure 6.  In the Sun’s absorption spectrum, the black lines indicate the elements that are present within the Sun, as they have absorbed photons of specific wavelengths.  The emission spectrum of the solar prominences can inform scientists about the speed and temperature of the solar prominence.  In addition the emission spectrum contains information about the magnetic field of sunspots.

 

Materials

 

  • Online simulation

Procedure

 

In this lab we are going to examine the blackbody spectrum of several objects. In Blackboard open up the Blackbody Spectrum Simulator. In the simulator check the boxes for graph values and labels. We can now analyze the spectrum of light produced by several sources. You may have to adjust the zoom function of the graph to accurately see all of the data.

Once you have a clearly defined image, take a screen shot of the spectrum. The screenshot should be clear and display all of the relevant data (It should look like the blackbody spectrum in Figure 2). In addition create a chart with the following data for each object: temperature, apparent color, primary wavelength of the radiation, max spectral power density, and other wavelengths of light produced in each portion of the spectrum. We will analyze, the Sun, Sirius A, a star at 8000K, Earth, and a light bulb. Repeat this procedure for each object.

Now that we have our blackbody spectrum of the object we will answer the following questions for each object:

  1. What is the primary wavelength of radiation that is given off by the object?
  2. How does this affect observational characteristics of the object (i.e. what color does it appear)?
  3. Are there any areas under the curve that fall outside of the visible spectrum? What are the implications of these regions? For instance if the Sun gives off ultraviolet radiation how does that affect Earth/life on Earth?

In addition you will answer the following questions:

  1. How did the color composition of the spectrum change with temperature? Give specific examples.
  2. How is it that the filament looks like the color of the sun, even though it is about 1/2 the temperature of the Sun? What would the peak wavelength of the sun be at half its temperature? Explain.
  3. How could a light bulb be made more efficient so it puts out more light in the visible?

 

Good Luck!

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