Build and Use a Simple Spectroscope

Subject Area: Physical Sciences

Grade Level: 5-8

Learning objectives for the activity
State science standards addressed by the activity
Safety awareness
The procedure for the activity
A student handout
Background discussion
A set of LIGO field trip activities
A downloadable MS Word version of the package

OVERVIEW

In this activity students will build a spectroscope in order to help them understand that white light is made up of a collection of colors from the spectrum and that different patterns of colored bands correspond to different sources of light. Our scope is inexpensive, but if assembled with care it can yield excellent qualitative results. Rather that cut a slit in a box by using a razor blade, we tape a pair of blades over a larger hole to make a very narrow and very uniform entrance for the light.
The activity presumes that students have been introduced to the visual light spectrum and to the concept of wavelength.

OBJECTIVES

Students will recognize that visible light is a collection of waves of different wavelengths, and that we perceive different wavelengths as different colors
Students will use a spectroscope to separate light into its various individual wavelengths
Students will define the term 'diffraction', and will describe the role of the diffraction grating in the spectroscope
Students will recognize that different types of light sources will produce different spectral patterns, and that the spectroscope can be used as a tool to identify unknown light-producing materials (such as chemical elements in stars)

STATE STANDARDS

SAFETY AWARENESS

Razor blades must be handled with care with a high level of adult supervision
Don't look at the sun or at laser light through the spectroscope

A MATERIALS LIST FOR EACH SPECTROSCOPE

A small cardboard box (roughly 20 cm on each edge would be fine)
Two flat single-edge razor blades (we used utility knife blades -- see safety tips)
Clear tape
A utility knife for cutting holes in the cardboard box
A diffraction grating. We used a 13,500-lines per inch grating from Rainbow Symphony, Inc. See www.rainbowsymphony.com Click the "Science and Education" button then the "13,500 lines per inch Diffraction Gratings" link. You can purchase 50 gratings for about $18 plus shipping, or you can buy 100 gratings for $30 plus shipping. There are many other sources of gratings on the Web.
A variety of types of light sources, both white and colored. You may also wish to supply some sheets of thin transparent colored plastic to use as filters in front of white sources.
A relatively nice day. Some clouds are acceptable, but light from the sky will look better in the spectroscope if the sun is out (see safety tips).

PROCEDURE FOR MAKING THE SPECTROSCOPE

Cut square holes in the center of two opposite faces of the box. The holes should be roughly 2 cm on a side.

Position the two razor blades flat on one face with the blades facing each other. The two blades should completely cover the hole you have cut.
	Now comes the tricky part. Slide a piece of paper between the blades that face each other. The idea is to have the blades separated only by the thickness of a single sheet of paper. One person should hold the paper upright once you are sure that the paper is completely between the blades.
With the thickness of the paper continuing to provide the only separation between the blades, tape the blades down to the surface of the box. Don't move the blades while doing so. Now pull the paper out from between the blades. The blade edges should now be parallel to each other over the hole in the box, and the edges should be separated from each other by the paper's thickness.
	If you now look into the box through the opposite hole (the viewing hole) with the blade side toward a light source, such as a fluorescent tube fixture in your classroom, you should see a thin vertical strip of light coming through the slit between the blade edges.
To use the spectroscope, either tape or hold the diffraction grating slide over the viewing hole in the box. Point the slit towards a light source and peer through the grating into the darkness of the box. You will need to position your eye close to the grating (The 'extra' slit and bands that you might see in the box are most likely due to subtle effects from the diffration grating).

A HANDOUT FOR STUDENTS

The Spectroscope

Name:

Now that you have made your spectroscope, use it to make as many observations of light behavior as you can. Try to find different sources of light. Fluorescent lights, incandescent bulbs in lamps, colored bulbs in lamps, candle flames, Bunsen burner flames, neon signs and the sky are all good potential sources. You can also try placing colored glass or colored plastic in front of a white source. Do not look at the sun through your spectroscope, and do not look at laser light through your spectroscope. Even though your viewing slit is narrow, both of these sources can still do permanent damage to your eye. When you observe a light source through the spectroscope, make a written observation of what you see, including a sketch.

Questions

Using your science textbook or other reference material to help you, define the term 'wavelength'.

Based on the results of this activity, do you think it is true that sunlight is made of all wavelengths of visible light?

3. The stripes of colored light that you see in the spectroscope are often called 'bands'. Are the bands from a blue sky skinny or wide? _______ How about the bands from a white fluorescent light?________ Can you explain why there is a difference?

If you used your spectroscope to look at a red light, like a stoplight, what do you think would be the appearance of the pattern in the scope? What could you say about the spectral pattern of a green light?

5. If all that you saw was a spectral pattern inside your scope - if you couldn't see the light from the source at all - could you say anything about the nature of the source? If, for example, you saw a spectral pattern of lines, could you say that the source was probably red, or probably green?

Astronomers use spectroscopy to help identify the chemical elements that make up distant stars. Using what you have learned in this experiment, describe how such identifications are possible.

Do some quick research (perhaps on the Internet) and see if you can find some ways that spectrometers are used in the 'real world'.

BACKGROUND DISCUSSION

Nature displays considerable variety in the types of waves that we can observe (waves can also be called oscillations or periodic behaviors).

Tidal cycles are oscillations that occur over hours; ocean waves occur in seconds; sound waves vibrate dozens, hundreds or thousands of times per second; light waves vibrate trillions of times per second.
Waves vary according to the materials (the 'media') through which they can pass (light waves won't pass through steel, but sound waves will).
We sense waves by a number of methods. Our eyes, ears and hands all detect different wave behaviors, and we have invented a huge number of devices for sensing waves - everything from seismometers to satellite dishes to X-ray film.

All types of waves, however, can be characterized by certain measurements. Wavelength is a measure of the length of a wave (crest to crest). Period is the amount of time that is needed for a single oscillation of the wave. Frequency is the number of oscillations that occur in a certain time (period and frequency are reciprocals of each other). The speed (or velocity) of a wave is how quickly the wave moves from one location to another. An important mathematical relationship that unites several of these quantities is

Wave speed = (wavelength) * (frequency)

Let's consider light waves specifically. What we call light is actually one portion of the electromagnetic spectrum, which includes radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays and gamma rays (in order of increasing frequency). All electromagnetic waves move at the 'speed of light' - roughly 300,000,000 meters per second (3.0 * 10⁸ m/s). Since all of these light waves move at the same speed, then those with higher frequencies must have shorter wavelengths, and vice versa.

Light from the sun is composed of all colors of the spectrum. We know this because when we pass sunlight through a prism, we see the colors of the rainbow. The prism refracts sunlight. Refraction occurs when the path of a wave is bent as the wave crosses a boundary into a new medium. Apparently, the different colors (waves) in the visible spectrum are refracted at different angles as they interact with the prism. We say that light undergoes dispersion in the prism (the prism disperses, or separates, the component waves).

Another way to separate the components of light is through the process of diffraction. Diffraction is the bending of a wave path that occurs as the wave passes through a narrow opening.

The diagram above shows the diffraction of a set of a waves through a single slit. If the light encounters not just one slit but many, many slits, such as on a diffraction grating, the waves will be diffracted by the slits. The diffracted waves undergo constructive and destructive interference with each other. The interference process will separate the waves according to their color, similar to the refraction process described above. You will use this fact to analyze different types of light when you use your spectroscope.

Back to Teachers Corner

This "cheap" adaptation of a spectroscope was developed by Fred Raab.

This page last modified January 26, 2005
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