University of Michigan - Department of Astronomy

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Principles of Spectroscopy

Star light, star bright.

Overview

Introduction

A photon is a small bit of electromagnetic energy sent across space. Photons can be emitted or absorbed by electric charges -- usually an electron.

Electrons absorbing and emitting photons

A hot, dense object contains many "loose" electrons which can emit photons of any energy. However an electron cannot emit a photon with more energy than the electron started with. The light produced by a hot, dense object is called thermal emission and it contains photons of all energies, i.e. light of all colors, or wavelengths. The resulting "rainbow" is called a continuous spectrum. As we heat up an object, we are giving the electrons more kinetic energy, so they become able to emit more energy. The hotter the object becomes,the brighter the continuous spectrum becomes. This is describedby the Stephan-Boltzmann Law:

f = σT4

As the emitting object is heated, the flux, f, of light energy emitted per unit area (the brightness) increases as the temperature, T (measured in Kelvin, K), to the fourth power; σ is called the Stefan-Boltzmann constant, and has the value 5.67x10-8J m-2 K-4. If two hot pokers are the same size, but one is twice as hot as the other, the hotter one will be sixteen times brighter. The same is true of two stars.

As the object heats up and the electrons get more energy, the energy of the typical photon emitted also increases. This means that the continuous spectrum gradually shifts toward shorter wavelengths (higher energies) and therefore looks bluer. This is described by Wien's Law, which says the peak wavelength times the temperature is constant:

λpeak * T = 0.29 cmK

which means that as the temperature, T, of the emitting object increases, the wavelength λpeak where the intensity of the light is the greatest must decrease. A very hot poker will glow with a bluer (shorter wavelength) light while a cooler poker will glow with a redder light.

Continuous Thermal Emission -- Blackbody Curves

Any hot, dense, opaque object can and must produce continuous spectrum across all wavelengths, with the total energy and dominant color described by these two laws. This is sometimes called blackbody radiation or thermal radiation. The object has no choice -- if it's hot, the electrons have energy, so they must emit light. Remember, Wien's law and the Stefan-Boltzmann Law apply only to continuous thermal emission.

So far we've talked about processes involving "loose" electrons that lead to thermal radiation. What about electrons that are part of an atom? In the Bohr model of the atom, electrons orbit a nucleus of protons and neutrons. Each orbit has a different potential energy, just like planetary orbits correspond to particular gravitational potential energies. But according to quantum mechanics, the electrons can only orbit in certain places, which means the electrons can only have certain orbital energies -- these allowed energies are called energy levels.

Energy level schematic

Electrons usually stay in low energy levels, but they can "jump up" to higher energy levels by absorbing a photon or by gaining energy in other ways. If it gains energy by absorbing a photon, it has to have exactly the correct amount of energy -- it has to match the energy difference between the energy levels. Therefore, the atom can only absorb light at a few specific energies, or colors. This is called line absorption. Line absorption occurs when a low-density gas is in front of a hotter, continuous spectrum source. The cooler, low-density gas acts to block the photons which have the right wavelengths, while the other photons travel through the gas unperturbed. This leads to a generally bright spectrum, with dark lines at specific wavelengths. The missing colors are called spectral absorption lines and result in an absorption line spectrum.

The energy-level jumping can also happen in reverse. The electron can "fall down" from a higher energy level to a lower one, emitting a photon with energy equal to the difference between the levels. This is called line emission, because photons are emitted. The spectrum produced is a set of bright emission lines, so it is called an emission line spectrum. This can only occur in a low density gas viewed on its own or in front of a cooler background (if a hot, dense object is in the background, we see line absorption instead of line emission).

Notice that these two processes only involve photons with particular energies that match its energy levels. Since each atom or molecule has a different set of energy levels, each atom or molecule also has a unique set of spectral lines.

Let's summarize what are known as "Kirchoff's Laws." First, a hot, dense gas (or a solid or liquid) has free electrons and will emit a continuous spectrum, with the brightness and typical color described by the Stefan-Boltzmann and Wien Laws. Second, a low-density gas along the line of sight to a hotter continuous radiation source will absorb photons of specific energies, leaving an absorption line spectrum. Third, a low-density gas viewed alone or in front of a cool background will produce an emission line spectrum.

As photons travel outwards from the center of the sun, where the density and temperature are high enough to allow fusion, they are constantly absorbed and re-emitted by the atoms in the sun.  Eventually they get to the outer edge of the sun, called the photosphere, which is where the sun changes from being opaque to being transparent. The photospere, then, is the layer where all the photons we see originate. The transparent region above the photosphere is called the atmosphere of the sun and has two major layers. The cooler thin layer abover the photospher is the chromospher. Above that is the increadably hot and thin Corona.

One photon by itself can't tell us much about the photosphere or atmosphere, but by looking at all the photons together, astronomers can gain information about the temperature, density, and chemical composition of the sun. This is done by looking at the spectrum of the light -- the number of photons (i.e. the brightness) at each wavelength.  Similarly, the characteristics of the spectra we will look at in the lab will tell us information about the sources of light we will use.


Continuous Spectrum

In the photosphere (and regions deeper in the sun), the density is so high that the gas is opaque. This area produces light with a continuous spectrum.  It is radiating simply because it is hot.

For this part of the lab, you will need to get a dispersion grating from your instructor. A dispersion grating does the same thing as a prism: it splits up the light into individual wavelengths so that you can see the spectrum.

Turn on the power supply and the lamp.  Adjust the dial until the light is comfortable to observe.  DO NOT TURN IT OVER 120 V! Hold the grating so the arrow points left and right and look through it toward the light bulb. If you don’t have an arrow on your grating, check with your GSI.  You can see the "first order" spectrum to either side of the filament. If you look further away from the filament, you should see another spectrum similar to the first. This is called a "second order" spectrum. Still further away from the filament will be the third, fourth, fifth and so on. Anything above the first order is considered a higher order spectrum. Note that there is a mirror image of the spectra on the other side of the filament, so there are 2 first order spectra, one to the right and one to the left of the filament. Watch out for spectra from other sources, such as other lights or even reflections inside the light bulb. Once you've figured out which spectra you want to observe, answer the following questions.

  1. What kind of light-source are you looking at: thin gas, opaque gas, solid, or liquid (circle one)?  According to Kirchoff’s laws, what type of spectrum should this produce?


  2. Observe the first order spectrum with the diffraction grating.  What kind of spectrum is it: continuous, line emission or absorption (circle one)?  How did you identify it as this type of spectrum?





  3. How many higher order spectra can you see? List at least two differences between the first order and higher order spectra.





The temperature of the filament can be changed by changing the voltage going to the light bulb. Start with the light on the lowest voltage necessary to see the spectrum, then slowly turn it all the way up to 120 V. Watch the spectrum as you turn it up, especially the relative strengths of the colors. The brightest color is the peak color and is generally the color of the filament. HOWEVER, our eyes and brains adjust quickly to the light, dimming the brightest colors and reacting poorly if the light is bright (so the filament will never look blue-green).  Because of this, you must take your first impression from the spectrum. The best thing to get the peak color is to close your eyes for a few seconds then take your first impression from the spectrum.

Change the voltage a couple times and observe the light, then answer the questions below.

  1. Was the bulb hotter at a low voltage or a high voltage (circle one)?

  2. At what voltage was the bulb brightest: high or low (circle one)?

  3. List the colors you could see at:
    1. high voltage



    2. low voltage



  4. Circle the color in your lists above that appeared to be the peak at high and at low voltage.
  5. As you increase the voltage, how do the colors change (overall and the peak color)?






  6. Use your observations of the light bulb to explain which would be hotter, a red star or a blue star? You must have at least 2 pieces of evidence.









  7. Explain using both Wien's and the Stephan-Boltzmann Laws: if two stars are the same size, which is brighter, a red star or a blue star?










Line Emission

Now we will look at "discharge tubes," which are each filled with a low-density gas made of a single kind of atom. Running an electric current through the discharge tube gives the electrons energy and kicks them up to a high energy level. The electrons quickly fall back to their original energy level, giving off a photon with a wavelength determined by the difference in energy between the levels. Most of these photons leave the gas without interacting with other atoms allowing us to view them. This is similar to the process that occurs in the low-density, incredibly hot outer most regions of stars called the corona and in low-density, gas clouds in space called emission nebulae.

Fist look at the tube with the power off and note where there are opaque solids.  Then turn on the power and observe where the light is actually emitted.

  1. What kind of light-source are you looking at: thin gas, opaque gas, solid, or liquid (circle one)?  According to Kirchoff’s laws, what type of spectrum should this produce?

There should be a spectroscope set up to observe the spectrum.  A slit is aligned with the light source that allows light to travel down to the diffraction grating at the eyepiece.  The spectrum is projected onto a scale to the left of the light source. Observe the spectrum through the spectroscope. 

  1. What kind of spectrum is it: continuous, line emission or absorption (circle one)?  How did you identify it as this type of spectrum?

  2. Observe the spectrum of one of the discharge tubes. Roughly sketch what you see, labeling the element's name and the colors of the brightest lines. Compare these to the chart of emission lines in the classroom.  PLEASE TURN OFF THE DISCHARGE TUBES WHEN NOT IN USE (but leave the sodium lamp on)

    element:

    emission chart
  3. Before leaving compare your drawing to the drawings of the different elements from the others in your class. How could astronomers use emission lines from an object?

  4. The hydrogen atom has only one electron, but when you look through the spectroscope, you see several emission lines.  Explain how this is possible (how are several lines generated and why can you see all of them if there is only one electron per atom)



Light Questions

  1. Describe two ways astronomers could determine the composition of a planet’s atmosphere without leaving Earth (hint, emission and absorption lines are not restricted to the visible spectrum.)

  2. Notice that the discharge tubes have different colors to our eyes. Can we use Wien's Law to tell the relative temperatures of the gas within the tubes? Explain.

  3. Calculate λpeak for the following stars, and estimate which color dominates the visible light.

    Star Temperature λpeak Color
    Sirius 10,000 K    
    Sun 5,800 K    
    Betelgeuse 3,000 K    


  4. The interior of the Sun is opaque, so none of the light emitted inside makes it directly to us. The region where the Sun changes from opaque to transparent is the photosphere. This is the visible surface of the Sun. Above that is the solar atmosphere, which has two layers, the chromosphere and the corona. The density and temperature of each of theses regions is given below. Use that information and Kirchoff's laws to label the diagram below with the type of spectrum that corresponds to each region, and explain how you know which type of spectrum applies to that region.
    Bottom of Photosphere: temp: 5800 K; density: 2x10-4 kg/m3
    Bottom of Chromosphere: temp: 4500 K; density: 5x10 -6 kg/m3
    Corona (average): temp: 106 K; density: 10-12 kg/m3
    Solar structure

updated: 8/30/07 by SAM

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