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Color and Spectrum
Document ID: 09_07_05_1
Date Received: 2005-09-07 Date Revised: 2005-10-21 Date Accepted: 2005-10-24
Curriculum Topic Benchmarks: S11.3.2, S11.3.6, S12.3.3, S10.4.1, S10.4.2, S11.4.6,
S11.4.7
Grade Level: Middle & High School [6-12]
Subject Keywords: light, color, spectrum, spectral analysis, blackbody
Rating: Advanced

Color and Spectrum

By: Stephen J Edberg, Jet Propulsion Laboratory, California Institute of Technology,
4800 Oak Grove Drive, M/S 301-486, Pasadena CA 91011
e-mail: Stephen.J.Edberg@jpl.nasa.gov

From: The PUMAS Collection http://pumas.jpl.nasa.gov
©2005, Jet Propulsion Laboratory, California Institute of Technology. ALL RIGHTS RESERVED.

Human beings color vision allows us to distinguish both large and subtle differences
between objects of similar color. Consider a forest, with its multitude of greens. But
objects with similar colors are not necessarily the same, as the jade and seaweed found on
some Pacific coast beaches illustrate. Researchers, chemists, criminalists, and many
other investigators study and compare objects and learn about their compositions by
breaking the light down into its composite colors, a technique called spectroscopy
(pronounced spek-TRAH-skah-pee). Most people are familiar with a natural presentation
of the spectrum (plural: spectra) of the Sun: we call it a rainbow.

In this lesson, students will examine the spectra of three sources of light that have very
similar colors. But these sources generate their light by three different mechanisms
electrons changing levels in individual atoms, electrons changing levels in a bulk solid,
and a bulk solid heated to incandescence (by electrons passing through it) whose light is
then filtered. Light generation is discussed in more detail below in Appendix 1,
Underlying Principles.

The study of spectra, whether from sources that emit light or objects that reflect light, is a
very powerful technique. It is used for learning about the compositions and sometimes
even the textures of objects, even when they are studied at astronomical distances.

Extensions to this lesson are found in Appendix 2.

OBJECTIVE: Demonstrate that similar-appearing lights can be distinctly different,
suggesting that the light emitted is generated in different ways.

APPARATUS: Three different sources of orange light are recommended for this
demonstration. All can be purchased at department or electronics stores.

(1)
A small neon hallway light (sometimes called a guide light). These
operate from standard electric outlets and can be found at many discount department
stores for less than $5. If possible, remove the diffuser packaged around the neon bulb
(the diffuser absorbs a lot of the already-weak light generated by the bulb).

(2)
An orange holiday light fitted into a standard electric nightlight. Both
are available from many discount department stores for less than $10 total. The
diffuser/shade that usually comes with the nightlight will not be used.

(3)
An amber light emitting diode (LED), current limiting resistor, and
suitable low voltage power supply (battery and holder or plug-in transformer). Solder the
LED and resistor in series and attach leads to the power supply. An electronics store
worker can help you find the right parts. The cost should be less than about $10,
depending on the parts purchased.

(4)
Transmission-type diffraction gratings. These are relatively
inexpensive and can be purchased mounted in 35 mm slide frames. Sources for gratings
include:

a) Rainbowsymphony.com:
http://store.yahoo.com/rainbowsymphony/difgratslidl.html
,

b) learningtechnologiesinc.com:
http://www.learningtechnologiesinc.com/productCat52734.ctlg
,

c) scientificsonline.com:
http://scientificsonline.com/product.asp_Q_pn_E_3001307
or
http://scientificsonline.com/product.asp_Q_pn_E_3054509
, and

d) sciencekit.com:
http://sciencekit.com/category.asp_Q_c_E_439536
.

Choose a grating with 750 1000 grooves/mm; fewer gr/mm will work, but the colors
will not be as well separated, and differences between light sources may be harder to
discern.

Alternatively, you can purchase a sheet of diffraction grating material and slide
mounts, from a camera store or on-line, to cut and mount the material yourself.
Make sure you buy a single, linear grating. Some sources offer two-dimensional
gratings that generate spectra in a cross pattern or starburst pattern; these will prove quite
confusing to users. Mounted gratings cost from about $0.40 to $4.25 each, depending on
quality and type of mounting. Cost for an adequate setup should be less than about $40
all together.

The light sources should be mounted together so they are oriented along a vertical line.
This allows for easier, direct comparison of their spectra. Mask the neon light and
holiday light with black electrical tape, or mount them behind black-painted cardboard,
wood, or foam-core with suitable holes cut in it, so the visible parts of the bright, emitting
areas match that of the smaller LED in size and shape.
Set up the light source board as far from the first row of students as is practical.

PROCEDURE: Discuss with the students their assumptions about light and color and
their origins. Talk about lights in the classroom, traffic signals, streetlights, store sign
lights, party lights, campfires, and candles. Discuss the colors, heat generated, and
sources of the light.

Now darken the room and examine all three light sources at the same time. Start by
comparing the orange lights by eye, without using the diffraction grating. The students
should note any differences in the colors of the three sources.




This figure illustrates (a) the light sources and (b) their respective spectral
signatures. For this photograph, slots were made with electrical tape over
the circular holes normally used for visual studies so that the spectra
would be more distinctive. Observed from student seating, the circular
holes should be adequate for discerning the spectral differences.
(a) The left side of the figure shows the three light sources, neon,
amber LED, and orange light bulb (top to bottom) and their similar colors.
Even though there is little difference between them as recorded by the
camera, a careful visual examination of the actual lights shows slightly
different shades of orange for each.
(b) The spectrum of each source is displayed to its right. The different
appearance of each indicates a different source for the light emission.
Note the different textures of the spectra and their different extents
toward the green and red. Orange is not a pure color.


Once everyone has studied the sources alone, the gratings should be used. Orient them so
the spectra extend right and left. Students will see spectra going both right and left, and
may see more than one spectrum in each direction. Depending on the grating type, either
the left or right spectrum may be brighter. To make discussion easier, ask the students to
rotate their grating so that the brightest spectrum is to the right or left, as you specify.
(a)
(b)
Neon
Amber LED



Orange-Filtered Tungsten
Have the students describe what they see and offer their ideas for why similar-appearing
sources look so different when their light is spread out.

ACKNOWLEDGEMENTS: This publication was prepared by the Jet Propulsion
Laboratory, California Institute of Technology, under a contract with the National
Aeronautics and Space Administration.

Reference herein to any specific commercial product, process, or service by trade name,
trademark, manufacturer, or otherwise, does not constitute or imply its endorsement by
the United States Government or the Jet Propulsion Laboratory, California Institute of
Technology.

Appendix 1

THE UNDERLYING PRINCIPLES: All the light sources rely on the current of
electrons coming from the wall outlet and/or a battery to excite or heat the actual light
emitter within each source.

The Neon spectrum is created by the energy released when electrons, excited first by the
electric current to higher energy levels, drop back to less excited states within the neon
atoms. These levels can be thought of as steps in a ladder, though the separation between
steps varies. Each transition from a higher energy step to a lower step emits light, called
a spectral line, at a wavelength that depends on the separation between the steps. With
many excited atoms, many electrons are moving between steps having different
separations and a variety of wavelengths is emitted. Quantum mechanics explains that
for each type of atom, the relative positions of the steps are fixed, so distinct wavelengths
of light, characteristic of the atoms involved, are emitted. The fundamental equation,
discovered in the early part of the