Measuring Wavelengths of Light

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    The phenomena of spectrum has been known for centuries. As early as 1752, the Scottish physicist Thomas Melvill observed the spectrum from different elements. The key is that each element has its uniques spectrum, or "signature" so to speak. John Herschel suggested that each element could be identifies by its characteristic spectrum. This was all long before anyone understood the cause of spectra.

    Today we have an understanding of the source of the Balmer series in Hydrogen and the spectra of other elements. We have discussed it in class. If one excites a gas (here it is contained in a tube which electrically excites electrons in the atoms) the excited atoms emit a characteristic line spectrum This is called an Emission Spectrum. We can see these through a diffraction grating where each wavelength of light is reinforced at a particular distance from the center of the light. Basically, each color of light that comes from a particular electron transition in the atom, is bent a different amount as it goes through the grating. By measuring how much the light is bent we can determine its wavelength. Red light is bent differently than blue light and so on.
     
     

    Objectives:

    1. To determine the wavelngths of light of at least one source.
    2. To determine the frequency of the spectrum.
    We use a Spectrum Tube in this experiment. This particular tube has hydrogen gas in it and you can actually see the real spectrum. Not all lines are visible. The darker the room, the more you can see. But even in a poorly darkened room you can see three or four lines.

    The Helium Spectrum looks like this. Helium has a different atomic structure than hydrogen so it gives off different characteristic light. Every element has, so to speak, its own signature. This way, once we identify if, we are confident of know what element gave off the light.

    Apparatus:

    • 1 Spectrum Tube Power Supply
    • 1 Spectrum Tube
    • 1 Spectrometer
    Procedure:
       
       

    You will use a student spectrometer. Your instructor will show you how to hold it and look at light sources. It consists of a box with a diffraction grating and a slit through which light enters the box.
     

    1. Use the values of wavelength you measured and by comparing with known values on the chart in the lab, determine what element you observed.
    2. Find a typical % error in your values. % error is found by:

      3. % error = 100 X [true value - exp value] / true value
    1. Look at an illunminated incandescant bulb through the spectrometer. Be careful to aim the slit right at the bulb. You should see the full spectrum of colors, Red, Orange, Yellow, etc through Violet. Draw below what you see and make sure to relate them to the wavelengths on the spectrometer.
    2. What are the ranges of the spectrum? i.e., what colors are the longest wavelength and what value is that, and what color is the shortest wavelength and what color is that?
    3. Now look at a flourescent light (overhead in the lab.) How is it different from the one you just saw ? Draw what you saw:
    4. Again, like above, what are the ranges of the spectrum? How does this compare with the results of the incandescant bulb? Why?
    5. Now look at the spectrum tube your instructor put out for you to observe. Draw what you see:
    6. How is this similar or different than the spectra above?
    7. Discuss the results, comparing continuous, absorption and emission spectra.
    8. Which colors are the longer wavelengths?

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    9. Which colors are the short wavelengths?

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    10. Which color has most energy?

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    11. Complete a writeup of this lab, including a discussion, conclusions, recommendations, etc. You can do this on the back of this handout.
    Later on in the semester you will have a chance to do this when we study the SUN. Your instructor will caution you about not looking directly at the SUN, but you should be able to see an absorption spectrum of the Sun. These are called Fraunhofer Lines.
     
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