Spectral Analysis Lab
Learning Goals
1. To observe the spectra or component colors of light given off by
different substances when heated, and to use this property to
determine the chemical composition of an unknown sample.
2. To explore how spectral analysis may be used by astronomers to
learn more about distant stars.
BACKGROUND
If you have ever looked closely at stars, you may have noticed that they are not all the
same color. Scientists have found that they can learn a great deal about a star’s temperature and
composition by analyzing the light it emits. How is this possible? When white light passes
through a prism, it appears as a continuous band of colors ranging from violet to red. These
bands of color are referred to as the visible spectrum. Scientists have discovered that each of
these colors has a characteristic wavelength that can be measured using a spectroscope—an
optical device that separates visible light into its component wavelengths. When light passes
through a spectroscope, three types of spectra are produced: emission, or bright-line;
absorption, or dark-line; and continuous.
Individual elements give off light when they are supplied with enough energy (often in
the form of heat or electricity). The light produced does not contain all the colors in the
spectrum. Rather, when the light given off by the element is viewed through a spectroscope, we
see a series of colored lines. This is called a bright-line spectrum. No two elements produce the
same pattern. Therefore, by measuring the wavelength of light seen in the bright-line spectra,
you can often determine the element which is present. Dark-line or absorption spectra are
produced when light is passed through an element that is a gas like helium or hydrogen. The
gaseous element present absorbs the same wavelengths of light that are found in its bright-line
spectra. When viewed through a spectroscope, this leaves dark lines in the otherwise continuous
spectrum which represent the missing or absorbed wavelengths. The study of dark-line spectra is
of particular interest to stellar astronomers, since stars produce such spectra when elements and
compounds in their atmospheres absorb certain wavelengths of light.
Materials Required
1 Hand-held
spectroscope
Color pencils
Copy of the lab
Gas tubes
Electric source
Procedure
In this lab, you will use a spectroscope to observe the bright-line spectra of several different
gases. Make sure you are familiar with using the spectroscope before starting.
A. Take the spectroscope and look at the gas tube that has been illuminated by the
instructor. Remember, the slit of the spectroscope should be vertical and pointed towards
the lit gas tube. You should see a sharp spectrum displayed on the side wall of the tube.
Rotate the eyepiece, if necessary, to position the spectrum on the side wall.
B. On the data sheet, record the name and color of the gas tube once it is illuminated. Then
draw the bright-line spectra you see when looking through the spectroscope at the gas.
Use color pencils to label the bright lines you observe in their appropriate positions.
Remember, the bright-line spectra appear as narrow, brighter bands in the full-color scale
that you see.
Data
Gas
Color
Bright Line Spectra
Hydrogen
Helium
Chlorine
Nitrogen
Mercury
Xenon
Neon
Unknown
Post Lab Questions
1. From your observations, how do the bright-line spectra of the samples tested differ?
2. What color appears at the far left end of the visible spectrum? What color do you see at the far
right end?
3. How did the spectrum of the unknown gas compare to the spectra of the known samples?
Which gas do you think the unknown contains?
4. What do stars have in common with your gas samples?
5. Stars, like our Sun, emit large amounts of radiant energy. How might astronomers determine
the temperature and composition of stars using a spectroscope?