Laboratory Techniques I, CHY 114
Chemistry Department, University of Southern Maine

 

Left: Emission spectroscope.
Center: Spectroscope aimed at emission tube.
Right: View through eyepiece, looking to the right of emission tube.

Emission Spectroscopy: What Causes the Color of the Aurora?

Solar windstorms occasionally provide the colorful show known in the northern hemisphere as aurora borealis. The colors of the aurora are the result of emission of light from atoms that have been excited by the energy of the solar wind. In this experiment, you will examine the processes that underlie color production by excited atoms.

Assignment

In your class text, use the index to look up and read about these concepts:

  • Bohr's theory of the atom
  • Bohr's equation describing energy levels available to the electron in hydrogen (Equation 7.5 in Chang)
  • electromagnetic radiation
  • color
  • emission spectra
  • continuous and discontinuous spectra

Goals

  • To calibrate a spectroscope using known emission wavelengths of mercury gas
  • To use the calibrated spectroscope to measure emission wavelengths of hydrogen gas
  • To determine the energy associated with each emission wavelength of hydrogen gas
  • To determine the electronic energy levels involved in the observed emission from hydrogen gas
  • To identify elements in common light sources, such as fluorescent lights and street lamps, by their emission spectra
  • To identify elements in the atmosphere of the sun, by emission of sunlight reflected from clouds or the moon

Overview

  • In the lab, you will record wavelengths for emission of light from mercury gas and hydrogen gas, using a bench-top spectroscope.
  • Also in lab, you will also use a hand-held spectroscope to look at spectra from available light sources.
  • Outside of lab, you will calibrate the bench-top spectroscope by graphing spectroscope readings for mercury against known wavelengths of mercury emission. You will then attempt to correlate the hydrogen wavelengths with electronic energy levels of the hydrogen atom.
  • Also outside of lab, you will try to identify the elements responsible for the light observed using the hand-held spectroscope.

Preparing for Lab

The following problems require calculations similar to those called for in the report on this experiment. Learn how to work these problems, showing your calculations with units. For calculations, answers are provided. Similar questions may appear on your prelaboratory quiz. For more guidance, look at your text, and at the Report Form for this experiment.

  1. Calculate the energy, in J, of one photon of visible light of wavelength 550 nm. ANSWER: 3.62 x 10-19 J
  2. Calculate the energy, in kJ, of one mole of photons at 550 nm. ANSWER:218 kJ
  3. Use Equation 7.6 (Chang, 9th edition) to calculate the quantity of energy emitted when the electron of the hydrogen atom drops from level 5 to level 3. ANSWER: 1.55 x 10-19 J
  4. Calculate the wavelength (nm) of the photon of question 3. ANSWER: 1280 nm
  5. Is the photon emitted by the atom of question 4 a photon of UV light, visible light, or infrared light? Visible light is 400-700 nm. Shorter wavelengths are UV, longer wavelengths are IR.

Procedures and Report

Download, print, and study the Procedure and Report forms for this experiment. Bring it with you to lab. You will write data and observations on the form during lab, as you carry out the Procedure. After lab, you will complete the Report Form by carrying out calculations to give your final lab results. These calculations include graphing data with Excel, so if you need to use USM computers for this purpose, be sure to allow time to visit the USM computer lab for this work.

Click to see the chart, Emission Spectra of Various Elements, which is similar to Chang, Figure 7.8 and to the chart in the chemistry lab.

CHY114 Lab Manual