**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.

![](media/image1.png){width="5.989583333333333in"
height="1.6777777777777778in"}

> 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.

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]{.underline}**

**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 twentieth
century by Max Planck, is usually written simply as E = h. Here, E is
the energy of the electron's transition, is the frequency of the light
emitted, and h is a constant called Planck's constant in honor of the
discoverer.

In a light emitting diode, electrons within the solid material of the
LED also drop from a higher level of excitement to a lower level.
Instead of having the multiple, distinct, narrow energy levels of
gaseous neon, which limit the wavelengths of light that can be emitted,
the complex LED material has a broad upper level and a broad lower
level, like two ramps, one above a curb and the other below a curb.
Electrons in the LED can jump from any part of the higher ramp to any
part of the lower ramp, permitting the emission of light at all possible
wavelengths for transitions between the range of energy states within
the higher level and the range of states within the lower level.

The light bulb spectrum is emitted by a hot metal, tungsten, with a
color pattern filtered by the orange paint on the glass envelope
surrounding it. The emission process, also explained by Max Planck, is
totally different for this source; it relies on the bulk properties of
the atoms in the metal to spread light across the spectrum. The orange
paint absorbs much of the blue-end radiation.

**[Appendix 2]{.underline}**

**EXTENSIONS:** (1) Set up a clear night light bulb with the same
wattage as the orange bulb. Use a dimmer or potentiometer (from a
hardware or electronics store) to control the voltage passing through
the filament. Have the students look at the bulb and its spectrum at
full brightness. Ask them to predict what they will see when the voltage
is reduced so the filament glows dull orange, matching as closely as
possible the orange holiday light. With the room very dark (since the
filament will not be bright when it glows orange), reduce the voltage so
the clear bulb's filament glows orange. Have the students compare this
with the other orange light sources.

This new source will exhibit a full, near-blackbody spectrum spanning
violet to red, unlike the orange light bulb, which shows no violet or
blue and little green. Even though the orange-filtered bulb at full
voltage, inside its envelope, produces large amounts of green, blue, and
violet, the orange paint on that bulb's envelope specifically absorbs
light with wavelengths shorter than the green that is seen. The clear
bulb at full voltage shows what the orange bulb's filament emits before
it passes through the orange paint filter.

\(2\) Have students observe lights around them at night. Storefront
lighting exhibits a wide variety colors and sometimes of spectra.
"Fluorescent" lights (which are really phosphorescent lights) can now
generate colors besides the "cool" (light blue tinted) white that is
often used for room illumination. Internally, all fluorescent lights
produce the same mercury emission spectrum of traditional fluorescent
bulbs (it is visible with a diffraction grating and very different from
neon), but a continuous spectrum of light is broadcast as well thanks to
the internal phosphorescent coating. The general color of the continuous
spectrum from these lights is determined by the phosphor coating on the
interior of the tube. Some of the colors now generated by fluorescent
lights were once generated with neon and other gases.

Streetlights generate light using high pressure sodium, low pressure
sodium, mercury (compare the emission pattern with fluorescent lights),
and tungsten. For daylight scouting in advance, golden yellow
low-pressure sodium streetlights are built with a rectangular luminaire,
whereas the "cobra head" luminaire is more commonly used with
orange-pink high pressure sodium or pale blue mercury streetlights. It
is interesting to compare the golden yellow color of low pressure sodium
streetlights and the similar yellow shade of a tungsten traffic signal
(the newer LED traffic signal's shade of yellow may be different). This
comparison will demonstrate great differences in their spectral
signatures, just as the neon guide light and orange holiday bulb do.

\(3\) The orange sources in this lesson and the yellow of low pressure
sodium street lights and traffic signals are convenient coincidences.
Other light sources, especially LEDs and filtered tungsten (colored by
\[a\] the bulb envelope's coloring, \[b\] with filters used for
photography, \[c\] made from cellophane wrapping plastic, or \[d\]
plastic report covers) expand the sources that can be compared. Some LED
night lights shift colors over a cycle a few tens of seconds long and
are very instructive in studies of color.

\(4\) A hardware store torch or a Bunsen burner will provide a clean
flame suitable for studying the emission spectra of some elements. Place
a little of the chemical in a wire loop (or on a screwdriver blade) and
insert it into the flame. The flame will change color and spectrum.
Chemicals to try, [individually]{.underline}, include:

Copper chloride, CuCl~2~ -- mixed spectrum; a good light show

Lithium chloride, LiCl -- good red spectral line with some continuum

Sodium sulfite, Na~2~SO~3~ -- strong yellow spectral line (sodium is
also often seen by placing any metal with sodium contamination in a
flame, including a common screwdriver)

Strontium nitrate, Sr(NO~3~) ~2~ -- yellow, orange, and red spectral
lines

\(5\) Measurements of color extents and spectral line wavelengths can be
quantified with tabletop spectroscopes commonly found in high school
laboratories or with handheld spectroscopes. Visit these vendors for
suitable apparatus:

a\) learningtechnologiesinc.com:

<http://www.starlab.com/psprod.html> (about 1/3 down the web page,
cardboard and plastic handheld spectroscope options)

b\) sciencekit.com:

<http://sciencekit.com/search.asp?t=ss&ss=spectroscope&c=0&x=8&y=10>
(for a variety of handheld and tabletop spectroscopes)