Download Emission Spectra

Objective: The purpose of this laboratory assignment is to explore the visual electromagnetic spectrum using a
spectroscopes and gas tubes.
Waves in the electromagnetic spectrum vary in size from very long radio waves the size of buildings, to very
short gamma-rays smaller than the size of the nucleus of an atom.
http://imagers.gsfc.nasa.gov/ems/waves3.html
Electromagnetic waves can not only be described by their wavelength, but also by their energy and frequency?
All three of these things related to each other mathematically. This means that it is correct to talk about the
energy of an X-ray or the wavelength of a microwave or the frequency of a radio wave.
The electromagnetic spectrum includes, from longest wavelength to shortest: radio waves, microwaves,
infrared, optical, ultraviolet, X-rays, and gamma-rays.
Visible light/ optical waves are the only electromagnetic waves we can see. We see these waves as the colors of
the rainbow. Each color has a different wavelength. Red has the longest wavelength and violet has the shortest
wavelength. When all the waves are seen together, they make white light.
The activity
1. Hold the hand-held spectroscopes to your eye. View the incandescent source through the spectroscopes.
2.
Record observations.
3.
View the gas tube sources that produce a discrete spectrum
4. Draw the spectra you see and explain how the spectra produced by the gas tube differ from the one
produced by the incandescent light.
5. Describe any relationship that might exist between the colors viewed in the spectrum and the appearance
of the light source to our eyes
6.
Consider electromagnetic fingerprints and discuss why they are important in identifying individual
elements.
7. Use the following chart to help you identify the various elements
8. Describe how you would use the previous investigation to determine what elements might be burning
from the flame colors or what elements a star might be made of.
Data / Conclusions:
Use colored pencils or crayons to complete the following assignment.
1. Draw the colors of the spectra you see in Incandescent Light
3. Explain how the spectra produced by the gas tube differ from the one produced by the incandescent
light.
4. Draw the colors of the spectra you see in the gas tubes.
5. Describe any relationship that might exist between the colors viewed in the spectrum and the appearance
of the light source to our eyes
6.
Consider electromagnetic fingerprints and discuss why they are important in identifying individual
elements.
7. Identify the various elements.
Teacher’s Page
PA Assessment Standards:
Physical Science, Chemistry and Physics
3.4.7A Describe concepts about the structure and properties of matter.
3.4.7B Relaate energy sources and transfers to heat and temperature.
3.4.7D Describe essential ideas about the composition and structure of the u niverse and earth’s
place in it.
3.4.10D Explain essential ideas about the composition and structure of the universe.
Technological Devices
3.7.7 Use appropriate instruments and apparatus to study materials.
3.7.10 Apply appropriate instruments and apparatus to examine a variety of objects and processes.
Preparation:
Show students the incandescent light source, the black light source, and one of the gas tubes other than
neon. (To work with gas spectrum tubes, follow the manufacturer’s directions. Gently insert the tube
into the power supply. Then, briefly turn the power supply on to illuminate the gas. Turn the power
supply off immediately after student viewing to prolong the life of the tube.) Briefly turn on each
source so students can see the color of the light, while instructing students not to stare directly at the
light sources for long periods of time. Turn each source off after it has been viewed. Ask students to list
three to five questions they have about what they see. Discuss that they will use the diffraction gratings
(or spectroscopes) to view the light from each source and ask them to predict whether each light
source’s spectrum will be similar to or different from that of the overhead projector
Students should reconfirm that white light can be diffracted into a continuous color spectrum as was
demonstrated at the beginning of the activity.
Follow directions provided with the spectroscopes and gas tubes and power source!
The electromagnetic spectrum
Light, or electromagnetic radiation, comes in many forms. There are radio waves, microwaves, infrared light, visible light,
ultraviolet light, X-rays and gamma rays, all of which form what is known as the 'electromagnetic spectrum'
The electromagnetic spectrum is subdivided into seven regions according to wavelength. Each portion of the spectrum
interacts with matter in a slightly different way and is given a different name. From longest to shortest wavelength the seven
divisions are:
Radio (wavelengths greater than 0.3 metres)
Earth's atmosphere hides most electromagnetic radiation from space except visible light, certain infrared frequencies and
radio waves. For this reason, we can place radio telescopes on Earth's surface and radio astronomy was the first non-optical
study of radiation from space. A number of the most massive galaxies were found to be extremely powerful sources of radio
waves. Radio astronomy led to the discovery of pulsars which pulse regular radio emissions.
Microwaves (wavelengths between 1 millimetre and 0.3 metres)
Earth's atmosphere begins to shield radiation from us. The most important form of microwave radiation in astronomy is
called the Cosmic Microwave Background (CMB). Discovered in 1965, CMB comes from all parts of the Universe with the
same intensity. CMB became solid evidence for the 'Big Bang' theory, which predicted that the shockwave of the primeval
explosion would be still detectable. ESA's Planck mission will study the CMB and thus will be seeing the Universe as it was
almost at its beginning.
Infrared (wavelengths between 700 nanometres – 1 millimetre)
The primary source of infrared radiation is heat. The higher the temperature, the faster the atoms and molecules in an object
move and the more infrared radiation. The first infrared space mission was IRAS (Infrared Astronomical Satellite) which
detected about 350 000 infrared sources. Later, ESA's Infrared Space Observatory (ISO) made important studies of the dusty
regions of the Universe. ESA's Herschel mission will build on this work.
Visible (wavelengths between 400 – 700 nanometres)
Until 1945, most astronomy was optical. This meant studying a very small range of wavelengths. It is from these optical
wavelengths that most people derive their picture of the Universe, dominated by bright stars and galaxies. Visible light is
predominantly released by objects between 2000 and 10 000°C. The NASA/ESA Hubble Space Telescope has a powerful
optical telescope on board which enables it to take stunning photographs in real colour.
Ultraviolet (wavelengths between 10 – 400 nanometres)
As soon as observations from above the atmosphere became possible, the classical techniques of optical astronomy were
extended into the ultraviolet. The Sun and other hot objects are sources of ultraviolet radiation. In 1978, the International
Ultraviolet Explorer (IUE) was launched. IUE dominated ultraviolet space astronomy for nearly two decades. It generated
spectra showing intensities at different wavelengths from selected objects in the sky. Temperatures, motions, magnetism
and chemical composition are all discernible in the ultraviolet spectra.
X-rays (wavelengths between 0.01 – 10 nanometres)
Most of the observable matter in the Universe today is in a hot state, radiating short-wavelength radiation and X-rays.
Massive clouds of gas at a very high temperature fill the spaces between galaxies. Whenever a new star is formed, a
collapsing cloud of gas reaches temperatures sufficient for nuclear reactions to start, powering the star. Conditions in the
primeval Universe were very different - with only a few pre-existing molecules and no dust available for cooling, only the
most massive clouds could collapse. They would make not stars, but black holes. Theorists suspect that giant black holes
may have been among the earliest objects created in the Universe and would have produced X-rays. Two ESA missions,
XMM-Newton and XEUS are designed to observe these X-rays.
Gamma rays (wavelengths less than 0.01 nanometres)
Gamma rays from space are blocked by the Earth’s atmosphere – fortunately for us, because this powerful radiation is lethal.
Gamma-ray telescopes in space give evidence for the processes that made the Universe habitable. When a massive star has
used up its hydrogen fuel, it ends in a supernova explosion, emitting gamma rays. During this explosion, radioactive
elements are formed and ejected into space, decaying or combining to form the other elements. ESA's COS-B satellite (19751982) created a catalogue of gamma-ray sources. ESA's Integral spacecraft, launched in 2002, takes this work forward,
studying the phenomenon known as 'gamma-ray bursts'.
http://www.esa.int/esaSC/SEM0W1T1VED_index_0.html
Observations: Seeing in visible wavelengths
Even a casual glance into a clear night atmosphere reveals that at visible wavelengths,
stars dominate our surrounding sky. Visible light is the predominant electromagnetic
radiation released by objects with temperatures of between 2000 and 10 000°C.
Stars come in a variety of masses, with the vast majority containing between one
tenth and ten times the mass of our Sun. The different masses determine how
efficiently they generate energy and this gives rise to the surface temperature. Lower
temperature stars shine with red light and high-temperature stars are blue or white.
Being yellow, our Sun is a middle temperature star measuring around 6000°C.
Orion Nebula's Trapezium cluster
NGC 2264, Cone Nebula
All the stars visible to the naked eye are part of our galaxy, the Milky Way. However, thousands of millions of other galaxies
stretch throughout space and these are mostly studied using visible wavelengths.
The visible wavelengths are also the realm of the emission nebulae. They are glowing clouds of gas and often form some of
the most breathtaking objects in the Universe, most often appearing red. This color comes from the predominant emission
from nebulae, which is hydrogen gas.
Because of the nature of the structure of the hydrogen atom, when it releases energy it does so efficiently at a specific
wavelength of red light (656 nanometres).
These are mostly clouds of gas that surround young, massive stars but some emission nebulae are
the death shrouds of old stars. These are historically, but confusingly, called 'planetary nebulae'.
Exploding stars also create emission nebulae known as ‘supernova remnants’.
The most common way for astronomers to analyse visible light from a celestial object is to split it
into its constituent wavelengths to form a spectrum. Studying this allows astronomers to analyse the
composition and physical condition of the celestial object under consideration.
Eye-catching celestial
helix
If they are close to a star, dust clouds can reflect visible light. Such clouds usually create blue
patches in space, similar to the colour of our daytime sky.
http://www.esa.int/esaSC/SEMNFEX5WRD_index_0.html
The following activities are from FUSE, a joint project of the National Aeronautics and Space
Administration and the Johns Hopkins University in collaboration with:
Centre National d'Etudes Spatiales (France), the Canadian Space Agency, the University of Colorado, and the
University of California, Berkeley.
http://fuse.pha.jhu.edu/overview/mission_ov.html
They have been reprinted with permission.
Exploring Our Universe:
From the Classroom to Outer Space
I. Spectroscopy
Activity #6
SPECTROSCOPY: CHEMICAL
DETECTIVE
NOTES TO THE TEACHER
Grades 8 and up
Level:
Objectives: Students will explain the process of dispersion, and will identify substances based on the visible
spectra they emit.
 Triangular prisms
Materials:
 Hand-held diffraction gratings
 Various light sources: (Note: if the light source is a long thin tube or filament, you can
avoid the need to put a slit in front of the source.)
o
o
o
o


Incandescent bulbs (continuous spectrum)
fluorescent tubes (coated tubes yield a seemingly continuous spectrum)
"black light" tube (uncoated tube for discrete spectrum)
spectrum tubes for different elements (discrete spectra)
Visible spectra for various chemical elements (see attached handout)
"Chemical Detective" activity (see attached handout)
Procedures: [NOTE: students should have previous exposure to the electromagnetic spectrum and to the
concepts of wave refraction and interference.]
1. Use a prism or diffraction grating with an incandescent bulb to project a continuous color
spectrum on a wall or overhead screen. Explain that the light is being dispersed, or
separated according to wavelength. Have students identify the colors present in the
spectrum; explain that there is no set number of colors in the spectrum, but that it is a
continuous range of colors.
2. Give students hand held diffraction gratings. Have them view an incandescent source to
reconfirm that white light can be separated into a continuous color spectrum. Next have
them view a source which produces a discrete spectrum (gas tube, "black light") and have
them explain how the spectrum they see is different. Ask them to describe any relationship
that might exist between the colors viewed in the spectrum and how the light source looks
to our eyes (white light has all colors, "black light" has purple, blue and green but not
much orange or red).
3. Ask students to consider fingerprints and explain why they are important. Tell them that
spectra can be used just like fingerprints: each chemical element and compound produces a
unique pattern of spectral lines. This pattern of lines can be used to identify the presence of
a particular element or compound in an unknown substance. Without telling them its
identity, illuminate a neon gas tube. Ask students to guess what is in the tube; have them
justify their guesses. Have students view the neon gas tube through their diffraction
gratings and record the number of spectral lines they view and the color of each line. See
whether students notice a relationship between the colors of the spectral lines and the color
of the light our eye sees (most of neon's emission lines in the visible range are red and
orange, so neon appears red). Give students a copy of the handout "Elemental Spectra"
showing visible spectra for various elements and have them match the lines they see from
the gas tube to the c
4. Arrange students into groups. Give each group a copy of the activity "Chemical
Detective." Have each group work as a team to solve the mystery and submit a written
report discussing their solution, the evidence they gathered that led them to the solution,
and how they used spectroscopic techniques to solve the crime.
Discussion: All secondary school physical science texts discuss the phenomenon of light dispersion. High
school physics texts usually include a mathematical description of the process by which different
wavelengths of light can be separated. Yet the texts seldom offer students activities that reinforce
these concepts. This is unfortunate, since a demonstration of spectroscopy techniques almost
always produces an "oh, wow!' response. The viewing of a spectrum- a rainbow of colors- is
always a memorable experience; it is also a vivid example of light's wave-like properties.
1. Give students diffraction gratings to take home. Have them observe light sources in their
Extensions:
neighborhood (street lights, business signs, etc.) For each light source they observe, have
them record whether the spectrum they viewed was continuous or discrete. For discrete
spectra, have students record the number of lines viewed, and the colors of the lines. Have
students use the sample spectra from various chemical elements to identify the
composition of the light source. (NOTE: there are charts listing the visible spectra for
typical light sources such neon, metal halide, sodium vapor and mercury- see Sources.
2. Discuss how a diffraction grating works. Develop the equation relating the wavelength of
colors in a spectrum to the dispersion angle and the spacing of the lines on the diffraction
grating [ = d sin ]. If available, use spectrographs to measure wavelengths of simple
spectral lines.
3. Have students who have learned triangle trigonometry do Activity #4 in Kit II: Tracing
Light Through - Understanding Diffraction. This activity teaches the principles on which
the FUSE spectrometer is based and tests understanding by asking students to trace light
rays from a star through the spectrometer.
4. Have students research the process by which rainbows are formed, and explain the
conditions necessary for viewing them.
5. Have students contact fluorescent light manufacturers (check websites like
www.sylvania.com or www.ge.com) to find out how fluorescence works and what
techniques they use to make fluorescent lights produce a spectrum similar to natural light
(sunlight). Also, they can investigate what type of lighting is most appropriate for different
situations, and how this relates to the spectral emissions.
Sources:
Flinn Scientific, 1-800-452-1261 (for hand held diffraction gratings)
Electro-Technic Products, Inc., 773-561-2349 (for gas spectrum tubes)
Arbor Scientific, 1-800-367-6695 ("Night Spectra Quest" chart for identifying typical light
sources)
CHEMICAL DETECTIVE
STUDENT ACTIVITY
You are a private eye who stays in business mainly by recovering lost dogs and the occasional runaway pet
turtle. You have just learned that last night some devious criminal elements pulled a huge bank robbery. This
could be your big break. Crack this case and you'll be famous!
You grab your diffraction grating and head to the scene of the crime. What is that strange glowing gas you see
in the bank vault? Hold your breath, it's a clue. You look at the gas through the grating. The emission lines you
see are like fingerprints. Now you can identify the criminal elements.
This is the "perpetrator spectrum":
Look at the visible light emission lines of the suspects, shown on the following page. Can you match the lines in
the "perpetrator spectrum" to the elements whodunnit?
The president of the bank is offering a huge reward to whoever solves this case but is demanding hard evidence
against the wrongdoers. Write a report to the president of the bank listing the names and aliases of the
perpetrators and, most importantly, explaining in detail how you used techniques of spectroscopy to crack the
case.
Cosmic Barcodes
Text by Ken Sembach and Bill Blair
Graphics by Ken Sembach
Astronomers learn about the Universe by observing light from distant astronomical objects, like stars or
galaxies. Light contains information, and since it is much easier to observe a star than it is to travel to one, there
is clearly a benefit to being able to understand what the light is telling us!
One of the main tools for studying light is a device called a spectrograph, which breaks light into its component
colors, much like raindrops refract sunlight to produce beautiful rainbows in the sky. When attached to a
telescope, a spectrograph becomes a powerful tool for learning about the Universe.
Luckily for astronomers, this technique can be used with all kinds of light, not just the visible light that our eyes
are sensitive to. Different kinds of light, like infrared, ultraviolet, X-ray light, etc., contain different kinds of
information. The FUSE satellite uses light in the far-ultraviolet spectral region, light having wavelengths
between 90 and 120 nanometers (1 nanometer = 1 billionth of a meter!). For comparison, visible light ranges
from about 400 to 700 nanometers (4000 to 7000 Angstroms).
In the absence of any intervening material, the light from a star reaches us unobscured. When the light is
dispersed into colors by a spectrograph, it may look something like this continuous spectrum, which has a
smooth, gradual change of color, and no breaks or dropouts in the intensity of the light:
However, if there are one or more gas clouds between us and the star, this interstellar medium absorbs some of
the light before it reaches us. Depending upon what types of atoms or molecules are present in the absorbing gas
clouds, a number of dark features, or absorption lines, are superimposed on the continuous spectrum emitted by
the star, something like this:
These absorption lines contain information about the composition of the clouds (the kinds and relative amounts
of atoms and molecules in the gas). They also tell us such things as how much gas is in the clouds, the gas
density, the temperature of the gas, how fast it is moving toward or away from us, whether there are cold
regions embedded in warmer material, and whether there are interstellar dust grains mixed in with the gas. All
this from analyzing the light!
These absorption features can be thought of as cosmic barcodes, with each type of atom or molecule producing
a different barcode signature. One can then think of a spectrograph as a "barcode reader". Once the barcode
produced by a gas cloud has been read, astronomers can interpret what the barcode means. Here are examples of
the types of barcodes that are produced by the FUSE spectrographs:
Now it's YOUR turn!
See if you can guess which elements have left their mark on this spectrum:
The Redshift Explained
The above diagram shows the spectrum (component colors) of a galactic star. The different wavelengths(the distance from
one wavecrest to the next) of light are what human eyes see as colors. The shortest wavelengths appear at the blue end of
the spectrum. The longest wavelengths appear at the red end of the spectrum. The numbers on the top measure wavelength
in nanometers. The deep black lines in the color spectrum are absorption lines. Each chemical element in a star's
atmosphere absorbs a certain color. By looking at the absorption lines in a spectrum, we are able to determine what
elements are present in a stars atmosphere (the missing colors are the colors that the elements in the stars atmosphere
absorb).
Notice how the pattern of absorption lines shifts from the blue end to the red end as the galatic star becomes fainter. This
is known as the Red Shift.
The Doppler Effect is used to explain the Red Shift. The Doppler Effect states that if a source that is emitting waves
moves away from us, the wavelength of the waves we recieve from it will be longer. This implies that stars moving away
from us will be red shifted(light with longer wavelength appears at the red end of the spectrum). In the 1920's, Edwin
Hubble discovered that many galaxies appeared red shifted. This means that these galaxies are moving away from us! The
universe is expanding! The idea that the universe is expanding supports the big bang theory.
Here are a few explainations of the Red Shift which do not support the big bang. *The following comes from
CREATION-EVOLUTION ENCYCLOPEDIA*
[1] Gravitational redshifts. Light rays from the stars must travel vast distances to reach us. It has been proven that the pull
of gravity, from the stars the light rays pass, could indeed cause a loss in light-wave energy—thus moving that light
toward the red on the spectrum. Einstein was the first to predict that gravity would affect starlight, and this was shown to
be true in the 1960s.—p. 35. Albert Einstein was the first to predict that gravity would be able to affect the transmission of
light. This fact could easily explain the redshifts which have been found.—p. 42.
[2] Second-order Doppler shift. It is known that a light source moving at right angles to an observer will be redshifted.
Compare this fact with the known fact that all stars are definitely circling galaxies. In addition, many scientists suspect
that, just as all planets and stars are kept in position by orbiting, so, for purposes of stability, the entire universe is
probably circling a common center!—pp. 35-36.
[3] Energy-loss shift. Light waves could themselves lose energy as they travel across the long distances of space. This is
called "tired light." The energy-loss shift is probably the primary cause of the redshift.—p. 36.
http://www.angelfire.com/nt/fairytales/redshift.html
BLUE SHIFT
The blue shift is a decrease in the wavelength of the light that is emitted from an object that is moving toward us. This
decrease in wavelength makes the object appear to be bluer than it actually is. For example, when a star is travelling
towards Earth, its light appears bluer (the light waves are shortened, shortening the wavelength). Compare with red shift.
http://www.allaboutspace.com/subjects/astronomy/glossary/indexb.shtml
Activity: Determining Red-Shift in a Receding Star
Instructional Objectives
Time Needed for Activity
Target Grade Level
Materials
Background Information & Questions
Web Resources
Instructional Objectives:
Students will 1.
2.
3.
4.
manipulate multivariable algebraic formulas,
understand velocity, wavelength and frequency,
study the Doppler effect,
determine the amount of red-shift in light from a receding star.
Time Needed For Activity:
45 minutes
Target Grade Level:
Advanced high school students.
Materials:


Calculator
Dictionary
Background Information and Questions:
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Part 1: LIGHT WAVES
Light is best described as a wave. All waves are characterized by a wavelength and a frequency.
The wavelength describes the distance between the crest of each cycle. The frequency is the number of crests
that pass any given point every second. Visible light waves range in wavelength from approximately 400
nanometers
(400 nm = 400 x 10-9m) for violet colors to about 700 nanometers for red. Correspondingly, violet has a
frequency of about 7.5 x 1014 Hz (where 1 Hz is equal to 1 cycle/second) and red has a frequency of 4.3 x 1014
Hz.
Light is a type of electromagnetic radiation. Other types of electromagnetic radiation like ultraviolet light,
infrared light, radio waves and X-rays also travel in the form of a wave but at wavelengths to which our eyes are
insensitive.
Sound is not electromagnetic radiation, but sound is a wave as well. Higher pitches are caused by higher
frequencies of vibrating molecules that reach your eardrum. Lower pitches are likewise caused by lower
frequencies.
Questions:
1.
2.
3.
4.
What color has the longest wavelengths?
What color has the shortest wavelengths?
What color has more crests of its wave passing a given point in one second? Explain.
Look up the prefixes "ultra" and "infra" in the dictionary. Explain why the wavelengths just out of the
visible spectrum are referred to as ultraviolet and infrared.
Part 2: DOPPLER EFFECT
If a wave source is moving, the crests of its waves get bunched together in front of the wave source. If the wave
crests are bunched together, their frequency increases. In the case of a sound wave, the pitch is higher. Behind
the wave source, the waves spread out and the pitch is lower.
In the case of light we use the terms "blue-shift" and "red-shift" to describe how the DOPPLER EFFECT
changes the wavelength of the light. Being blue-shifted or red-shifted doesn't mean that the light necessarily
becomes blue or red. It means simply that the light's wavelength either is shortened (blue shifted) because the
object giving off the light is approaching, or is lengthened (red-shifted) because the object is moving away from
the observer.
Part 3: DETERMINING THE COMPOSITION OF STARS
When energized atoms and molecules vibrate, they give off massless light particles called photons. These
photons travel as a wave, but because of quantum energy effects, a particular type of atom or molecule gives off
only certain wavelengths of photon light. For example, when hydrogen atoms are giving off energy in the form
of light, they emit light specifically at wavelengths of 410.2 nm, 434.0 nm, 486.1 nm, and 656.3 nm. This is
called the emission spectra of hydrogen.
Scientists can use the emission spectra of atoms and molecules to study the composition of stars. Scientists need
simply to look very carefully at the intensity and wavelengths of the light given off by the star. A star
containing hydrogen, for example, would have intense peaks of energy at 410.17 nm, 434.05 nm, 486.13 nm,
and 656.28 nm. These hydrogen emission peaks would be in addition to the ones associated with the other
elements contained in the star.
Part 4: RECEDING STARS
In the 1920's, Edwin Hubble, while studying the stars of distant galaxies, found that for some, their emission
spectra had peaks at 411.54 nm, 435.50 nm, 487.75 nm, and 658.47 nm. Hubble knew that these wavelengths
did not correspond to any known element and that it was not likely that a combination of other elements or
molecules was responsible. He did notice that these spectral lines corresponded to hydrogen's emission spectra
except that they were all 0.0033 percent longer in wavelength than they should have been for a hydrogen
spectra. Hubble deduced that this red-shift must be because of a Doppler effect. His calculations showed that
these galaxies must be moving away from earth at 1 x 106 m/s (one million meters per second)!
Part 5: VERIFY HUBBLE'S WORK THAT SHOWS THE UNIVERSE IS EXPANDING
For all waves, the product of wavelength and frequency gives the velocity of the wave where (lambda) is the
wavelength and f is the frequency.
v = (lambda) f
In the case of light and other electromagnetic radiation, however, the velocity is always fixed at 3 x 108 m/s.
This speed of light is assigned the variable c.
vlight = c = 3 x 108 m/s, or more precisely, c = 2.99792458 x 108 m/s
Use c = 2.9979 x 108 m/s for any calculations below. Also, use the appropriate number of significant digits.
Questions:
1.
2.
3.
4.
5.
What would the frequency be of a violet light wave with wavelength of 410.17 nm?
What frequency is associated with 434.05 nm?
What frequency is associated with 486.13 nm?
What frequency is associated with 656.28 nm?
Imagine the hydrogen atoms in one of Hubble's distant stars emitting a photon of light at a wavelength
of 410.17 nm. If that particular photon were headed directly TOWARD Earth, how much CLOSER to us
would that photon be after one second? (Recall that the distance, d, that an object travels in a time, t is
given by d = vt where v is the object's velocity.)
6. Now if the star that emitted the photon were traveling AWAY from Earth at 1 x 106 m/s, how much
FARTHER from Earth would the star be after one second?
7. After one second, what is the distance between our initial photon and its star?
8. Our initial photon would be followed by many, many more just like it, each behaving similarly to the
first. After one second, how many wave cycles of photons would stretch between the distant star and our
initial photon?
9. What is the average wavelength of this photon light? (Hint: Use your answers from 4 and 5 above.)
Compare this with what Hubble saw.
10. Convince yourself of the validity of this process by repeating questions 2 through 6 for one more of the
hydrogen emission spectra peaks at wavelengths 434.05 nm, 486.13 nm, or 656.28 nm. (Choose (a) the
distance the photon travels toward Earth, (b) the distance the star moves away from Earth, (c) the
distance between the photon and the star, (d) the number of cycles between the star and the photon, (e)
the effective red-shifted wavelength.)
11. If you were driving in a VERY fast car, how would the things you approach look different from normal?
How would the things you drive away from look different? Explain.
12. Why do you think Hubble's discovery that the universe is expanding would have been impossible
without instruments that could precisely measure emission spectra?
http://www.pbs.org/deepspace/classroom/activity2.html