Conceptual Physics Lab: Investigating the Visible Spectrum
Problem: How can you use electromagnetic waves within the visible spectrum to identify unknown gases?
Part 1: Investigating how light is produced
Directions: Open the PhET model, “Neon Lights and other Discharge Lamps”. In the options (lower
right on the model), check the spectrometer and squiggle settings. Notice the energy diagram on the right of the model, as well as the red arrow labeled “Energy at collision”.
1. Fire a single electron towards the hydrogen atom. Describe what happens in a step by step
fashion. [N.B. - It may be helpful to utilize the Run in Slow Motion option for this part.]
2. How can the “Energy at collision” arrow be manipulated in this model?
3. How is light “produced”, according to this model?
4. What role does energy play in the production of electromagnetic waves (light)?
5. Is it possible for a single electron to collide with the atom of hydrogen which results in more than
one photon (particle) of light being emitted? Explain how. [Utilize the slow motion option for this!]
6. When the hydrogen atom is moved all the way to the left in the discharge tube, it is not possible
to cause the emission of light. Why is this?
Directions: Click on the “Multiple Atoms” tab on the model. Make sure that the Spectrometer and Squiggle options are checked. For each of the available gases given in the drop down menu on the right side of the model, record the colors and intensities of light in the visible spectrum emitted by each element.
Hydrogen Emission Spectra Mercury Emission Spectra
Sodium Emission Spectra Neon Emission Spectra
7. Based upon your observations using this model, which colors of visible light are highest in
energy? Justify your response.
8. What is the relationship between frequency and energy, based upon your observations?
Wavelength and energy? Explain.
Part 2: Observing and Identifying Unknown Gas Discharge Tubes
Directions: Stationed around the room there are several gas discharge tubes. Visit each of the stations and view the gas through the spectroscope. In your lab report record the visible emission spectrum for each type of gas you are observing. Carefully diagram and record the colors you see (use colored pencils). Make note of the order, width, and intensity of the colors you observe. View an
incandescent light (classic light bulb) through a spectroscope. In your lab report carefully diagram and record the colors you see (use colored pencils). This diagram will serve as a reference for the visible spectrum.
Helium Emission Spectra Argon Emission Spectra
Neon Emission Spectra Air Emission Spectra
Mercury Emission Spectra Hydrogen Emission Spectra
Incandescent Light Bulb Emission Spectra Fluorescent Light Bulb Emission Spectra
Unknown A Emission Spectra Unknown A Emission Spectra
Identify each of the two unknowns:
Unknown A:_____________________ Unknown B:_____________________
1. Why do the colors of light in a spectroscope appear as lines? (Refer back to your part a.)
2. What is meant by the “visible” spectrum?
3. What is the order of colors (from lowest wavelength to highest) in the visible spectrum?
4. Why can the electromagnetic spectrum be used to identify the presence of certain elements? Explain, citing evidence from both parts of this lab experiment.
5. How did your results for hydrogen, mercury and neon compare to the PhET model? Note any discrepancies.
6. Which element is present in the fluorescent light? Explain in detail how you can use your data to decide which gas is present
7. Which of the gases that you tested are present in air?
Answering the Problem: