What is a Spectrum?

White light is made up of light of different colors.  You can see the different colors when white light shines through a prism.   A prism bends the light waves and separates them into red, orange, yellow, green, blue and violet light.  We call these colors the visible spectrum because we can see them.

Water drops bend light waves and separate them into the spectrum, too.  A rainbow is a spectrum made by sunlight shining through raindrops.  Other materials such as CDs and diffraction gratings can also be used to separate white light.

Astronomers study of spectra of light coming from stars, quasars and galaxies to learn about them.  By looking at spectra astronomers can find out what distant objects are made of, their temperature, and how fast they are moving.

Investigation One: Spectra of lamps

Differences in the spectra of stars give astronomers information about their temperature.  Hot stars have more blue in their spectra and cool stars have more red.  You can detect differences in the spectra of different kinds of lights with a spectroscope.


Spectroscope safety: NEVER LOOK DIRECTLY AT THE SUN!
  • Observe the spectrum of the Sun only by looking through a spectroscope at sunlight reflected off a surface such as the pavement or a wall.

1. Use your spectroscope to look at each light source, from a distance of about an arm's length.   Darkening a bright room makes it easier to isolate each light source.
2. Record the spectrum of each by drawing a color picture of what you see.

 * How do the spectra of these light sources differ?
 * What does this tell you about how the light is produced?


Investigation Two: Spectra of different atoms


Isaac Newton was the first scientist to study the spectrum of sunlight.  He thought the spectrum was "continuous" because the colors seemed to flow from one to the next.  As spectroscopes improved scientists were surprised to find that spectra are not always continuous.  They found gaps, or black lines that interrupted the band of colors.

In the 1800's a German scientist named Gustav Kirchoff studied the spectra of light produced when he heated different chemicals.  He discovered that every material makes it own unique pattern of gaps and colored lines.  These spectral lines are very important to astronomers because they can be used to identify the gases and materials in stars and clouds in space.   You can see the spectral fingerprint of some common elements with gas discharge lamps.


1. Use your spectroscope to look at the lamps of different elements
2. Draw a color picture of what you see.
3. Compare your drawings to the spectra of different materials shown in the key.

* How do the spectra of these light sources differ?
* Do you think the spectra of all stars are the same? Why or why not?

See the spectra of different materials http://home.achilles.net/~jtalbot/data/elements/index.html
Find out about spectral lines http://cfpa.berkeley.edu/Education/DEMOS/Desk_Top_Stars/LS_Background.html

Investigation Three:  The spectrum of the Sun


When the Sun comes up in the morning, light comes through your bedroom window.  If you want to sleep late, you close the blinds.  The room stays dark because the light coming through the glass is absorbed by the window coverings.  When you get up, you turn the blinds to let more light in.  Some light comes in between the slats, and some is blocked.  You see bands of light and shadow.  The shadows are gaps in the light.

The dark bands in a spectrum tell us that some of the colors in the light have been blocked. The light from a star must travel through layers of slightly cooler gases that surround it.  Gases in the star's atmosphere act like your bedroom blinds absorbing some of the light waves and letting others through.  Predict what the spectrum of the Sun will look like.

     * spectroscope
     * colored pencils or crayons

1.  Look at reflected sunlight on a wall or building.  DO NOT LOOK DIRECTLY AT THE SUN!
2.  Draw a color picture of what you see.

 * What does the spectrum of the Sun look like?
 * What does the spectrum tell you about the Sun?
 * What do you think the spectrum of the Moon will look like?

Learn more about the spectra of other stars.

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