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1
THE ELECTROMAGNETIC SPECTRUM
Objectives
This exercise will allow you to visualize the range of the electromagnetic spectrum so that you
can appreciate the width of all its different parts. In addition you will also use an animation to
see the differences between continuous, emission and absorption spectra. Lastly you will
observe spectral lines produced in the lab and identify the wavelengths of emission lines
formed by different elements.
Equipment
Ruler with centimeter markings, colored pencils.
The last page of this lab shows spectra from 5 elements. Please DO NOT print page 12 (last
page) as it will waste your printer’s ink! Simply view this page online to make measurements.
Introduction
The electromagnetic spectrum is the entire range of electromagnetic waves which are divided
into different regions named as radio, infrared, visible, ultraviolet, x-rays and gamma rays.
While all these waves travel at the speed of light (3 x 108 m/s) they do not have the same
wavelength or frequency. Recall that the equation relating speed (c), frequency (f) and
wavelength (λ) is c = f λ. Hence if any two variables are known, the third can be calculated. Any
one of the following equations can be used to find the unknown quantity:
c = f λ f = c/λ λ = c/f c = 3 x 108 m/s
When these equations are used, it is important to keep track of units. If speed c is measured in
meters per second (m/s), frequency will be in Hertz (Hz) and wavelength will be in meters (m).
For example, let’s calculate the wavelength of yellow light if its frequency is given as 5 x 1014 Hz.
λ = c/f = (3 x 108 m/s) / (5 x 10 14 Hz ) = 6 x 10-7 m
The answer above can also be written as 60 x 10-8 m or 600 x 10-9 m or 6000 x 10-10 m
There is a reason why we are introducing all these different exponents. It is inconvenient to
keep saying “10-7” so prefixes (shortcut words for the exponents) have been developed. The
prefix for 10-9 is nano abbreviated as n. Hence the wavelength of yellow light can be written as
600 nanometers or 600 nm.
Another term also used with electromagnetic wavelengths is the “Angstrom” abbreviated as Å
which is 10-10 m. Hence the wavelength of yellow light can also be written as 6000 Å.
The list below summarizes commonly used metric prefixes, their names and abbreviations.
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10-9 nano n
10-6 micro µ
10-3 milli m
10-2 centi c
103 kilo k
106 mega (million) M
109 giga (billion) G
1012 tera (trillion) T
10100 googol (yes! That’s the source of the word you are familiar with. Strictly speaking, this is
not a metric unit, but a whimsical word given by mathematicians.)
This lab will show another important property of electromagnetic waves, which is that the
range of each of the regions (radio, ir, visible, uv, x-ray and gamma ray) is not equal. Also, some
regions include many familiar terms that you may not connect to an astronomy course, so here
is an opportunity to learn how this course affects your daily life!
Spectroscopy is a very important tool for astronomers. Each chemical element has its own
distinctive finger print or bar code revealed by its spectral lines. Chemical compounds made up
of two or more elements will show lines from each element, and the width and brightness of
spectral lines gives additional information about the chemical constituents of objects. Since
light is the only information we get from the stars, it is through spectral analysis that
astronomers have figured out everything we know about stars, like their temperature, mass,
size etc.
When sunlight passes through a prism or a diffraction grating, it breaks up into its component
colors, which is the familiar band called a “spectrum.” In the nineteenth century scientists
learned to make many different types of spectra which were examined in great detail with
spectroscopes, which are instruments consisting of a prism or grating to produce the spectrum
and a small telescope to enlarge the colored spectrum and see its details. Three different types
of spectra are summarized below.
1. A continuous spectrum shows a continuous band of colors, red merging into yellow,
green and blue. It is produced by a dense gas or a luminous solid. You can easily see a
continuous spectrum if sunlight passing through a hanging crystal makes a “rainbow” on
a wall, or if you hold up the shiny surface of a CD to a light source.
2. An emission spectrum consists of a series of brightly colored lines, and each element
shows specific colors in specific positions. It is produced by a low density gas if it is
heated sufficiently. These types of spectra are easy to produce in the lab by passing
electricity at a high voltage through a discharge tube containing the gas. Scientists have
made accurate photographs of these types of spectra, and the position of the colored
lines has been measured very accurately and converted to give their wavelengths.
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3. An absorption spectrum looks like a continuous spectrum, but it has dark lines on it. It is
produced when a cool, low density gas is placed between the light source and the
spectroscope. It is noticed that the location of the dark lines depends on the nature of
the intervening gas. For example if the intervening gas is oxygen, the absorption
spectrum will show dark lines in the same position showed by an emission spectrum of
oxygen.
To understand how spectra are produced, recall that each chemical element has its own
number of protons, neutrons and electrons. The protons and neutrons are tightly bound in the
tiny nucleus and do not contribute to forming spectra. It is the movement of electrons which
produces spectra. The electrons in an atom are organized in different orbitals or shells and each
orbital has its own energy level. The energy levels are also like rungs on a ladder, so an electron
can be in level 1 or level 2, but not on level 1.5. The energy levels of electrons in atoms are said
to be quantized, i.e. each level has a discrete value associated with it. Also, the energy increases
the further away the electron lies from the nucleus, meaning that energy levels further away
from the nucleus have a higher value. Just as it takes energy to climb a ladder from rung 2 to
rung 4, an electron has to absorb energy to go from energy level 2 to energy level 4. Similarly,
just as you decrease your potential energy if you climb down from rung 5 to rung 2, the
electron decreases its energy if it moves from energy level 5 to energy level 2. The excess
energy between level 5 and 2 will be emitted as a photon. A photon is a tiny packet of
electromagnetic energy, or simply put a particle of light. The photon’s energy is related to its
frequency by the equation E = hf and frequency f is related to wavelength by
c= fλ .
Let’s first explain how an emission spectrum is produced. If oxygen gas is placed in a discharge
tube and a high voltage is applied to the tube, the electrons in the gas will be energized and
move to higher energy levels. But not wanting to stay there, they will move back to their
original energy levels and emit the photons they had previously absorbed. This produces an
emission spectrum with many colored lines at specific positions. Each line’s position indicates
its wavelength, which can be related to the frequency and the difference in the energy level of
the electron.
While an emission spectrum is produced by a gas at low density, a continuous spectrum is
produced by a luminous solid or a dense gas with millions more electrons available to do many
millions more chaotic jumps. This will give rise to many millions of photons at different
wavelengths. Think of the continuous spectrum containing all wavelengths compared to the
emission spectrum which contains only specific wavelengths.
If the light from a continuous spectrum passes through a cooler gas like oxygen, the oxygen
atoms will absorb their preferred photons. The result will be a continuous spectrum deficient in
certain photons, indicated by the dark lines. This is the absorption spectrum.
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It is noticed that absorption lines occur at the same wavelengths as emission lines since each
indicates the presence or absence of photons with a specific energy and frequency. Measuring
the positions of the dark lines indicates that the intervening gas was oxygen.
Pre-lab Questions
1. The speed of x-rays is
a. Faster than light
b. Slower than the speed of gamma rays
c. Same as the speed of radio waves
d. Same as the speed of seismic waves
2. Use the equation f = c/λ to calculate the frequency of radio waves whose wavelength
is 50 m.
a. 0.6 Hz
b. 6 x 106 Hz
c. 6 Megahertz
d. Both b and c are correct
3. Use the equation λ = c/f to calculate the wavelength of ultraviolet light whose
frequency is 2 x 1015 Hz.
a. 1.5 x 107 m
b. 1.5 x 10-7 Hz
c. 1.5 nm
d. 1.5 x 10-7 m
4. Which of the following units is the smallest?
a. Millimeters
b. Centimeters
c. Nanometers
d. Kilometers
5. The wavelength of blue light is 450 nm. This can be written as
a. 450 x 10-9 m
b. 4.5 x 10-7 m
c. 4500 A
d. All answers are correct
6. An emission spectrum shows
a. Brightly colored lines
b. A rainbow of colors merging into each other
c. Only red and orange bands
d. Only green and blue bands
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7. The spectrum of a star has dark absorption lines of helium superimposed on a
continuous spectrum. What can you conclude from this?
a. The star is made up of helium
b. There is a great deal of helium in the earth’s atmosphere
c. The dark lines have absorbed helium
d. The light from the star has passed through a cloud of helium
8. If an electron moves from a lower energy level to a higher energy level, what type of
spectrum will be produced?
a. Continuous
b. Emission
c. Absorption
d. All are possible
Lab Exercise
NOTE: Steps to be performed are in alphabetic order (A, B, C…). Questions to be answered are
in numerical order (1, 2, 3,…)
A. The table below lists the range of electromagnetic waves. Since each range merges
smoothly into the adjacent one, the values given are approximate. There may be some
overlap in numbers used depending on the source. Colors seen with the human eye are
also subjective, as some people may not be able to distinguish between shades of
yellow and orange or blue and green. The quality of printers, inks, and screen
resolutions also determines how accurately colors are evident.
Wave Type Frequency
(Hz)
Frequency
X 10 14 Hz
Wavelength
nm
Radio 1 – 1012
1 – 0.01 x 1014
Infrared 1012 – 1014
0.01 x 1014 to
1 x 1014
Visible Red
4.3 x 1014 – 4.8 x 1014
4.3 x 1014 – 4.8 x 1014
698 nm – 625 nm
Visible Orange
4.8 x 1014 – 5.1 x 1014
4.8 x 1014 – 5.1 x 1014
Visible Yellow 5.1 x 1014 – 5.4 x 1014
5.1 x 1014 – 5.4 x 1014
588 nm – 555 nm
Visible Green 5.4 x 1014 – 6.2 x 1014
5.4 x 1014 – 6.2 x 1014
Visible Blue 6.2 x 1014 – 7 x 1014
6.2 x 1014 – 7 x 1014
Visible Violet 7 x 1014 – 7.4 x 1014
7 x 1014 – 7.4 x 1014
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Ultraviolet 8 x 1014 – 1016
8 x 1014 – 100 x 1014
X-rays 1016 – 1020
100 x 1014 – 1,000,000
x 1014
Gamma rays > 1020
>1,000,000 x 1014
B. Take a look at the numbers in the data table shown in the previous step. The second
column lists all the frequencies, while the third column has the same frequencies
recorded in powers of 1014 . The fourth column has some wavelength values already
worked out, while some are missing. You will fill in the missing values later in the lab.
C. Next you need a printed copy of page 11 of this lab. This is called Chart # 1 and you will
use it to mark various parts of the electromagnetic spectrum, and also attach it when
you submit the lab.
D. The top line of the Chart shows 25 intervals with about 1 cm per interval. The exact
distance will depend on the resolution of your screen and printer. Notice how each tick
mark rises by a factor of 10.
E. Use the second column of the data table shown in step A to mark the location of as
many parts as you can on the chart. For example, from the left end up to 1012 will be
radio, from 1012 to 1014 will be infrared etc.
F. Label the chart clearly as you will be uploading it when you enter your lab answers.
G. You have constructed what is known as a “logarithmic” scale, where each centimeter
represents a power of 10. This is done to enable us to fit a wide range of numbers in a
small space.
H. Use the chart to answer the following questions:
9. Which of the following regions is the widest?
a. Ultraviolet
b. Visible red
c. Infrared
d. X-ray
10. On the first line were you able to mark separate lines for visible blue and visible red?
Give a reason for your answer.
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I. The radio part of the spectrum spans 12 orders of magnitude (1-1012) and you use many
sub-sections of this part of the electromagnetic spectrum daily. Mark the following
values on the line on your chart:
AM radio = 105 – 106 Hz
FM radio = 107 – 108 Hz
Cell phones = 109 Hz
Microwaves = 109 – 1012 Hz
J. You know that sunscreens help protect your skin from the harmful effects of ultraviolet
rays from the Sun. Sunscreen creams are often labelled as protecting from UVA, UVB,
UVC rays. These terms simply signify the various frequency ranges for which the cream
offers protection. Mark these on the chart if you can:
UVA = 7.5 x 1014 – 9.4 x 1014 Hz
UVB = 9.4 x 1014 – 10 x 1014 Hz
UVC = 10 x 1014 – 30 x 1014 Hz
K. To enable you to get another perspective, let’s write the frequency of all parts of the
electromagnetic spectrum in terms of one standard exponent, which we will choose to
be 1014. These values are shown in the third column of the table.
L. Use the middle line of the chart to mark the values expressed as x 1014 and shown in
the third column of the data table. Since each tick mark is 1 cm long, you will not be able
to mark the radio and infrared regions. But with a centimeter ruler indicate the red
region from 4.3 cm to 4.8 cm, and color this area red. Mark the rest of the visible colors
on your line and color the areas using colored pencils/crayons. Also mark the location of
whichever other bands you can, like UVA, UVB, UVC.
11. Can you mark the location of UVC, X-rays and gamma rays on the middle line of the
chart? Give a reason for your answer. How long should the paper be to plot the
position of gamma rays? Keep in mind that each notch on the middle line was 1 cm on
the chart.
12. The scale used on the second line was a “linear scale” where each centimeter was
equal to the previous and following segment. How is this scale different from the first
line where you used a logarithmic scale? What advantages do you see in each scale?
M. Using spectroscopes it is possible to actually measure the wavelength of spectral lines.
E.g. if the red frequency (f) is 4.3 x 1014 Hz, the corresponding wavelength (λ) is:
λ = c/f = (3 x 108 m/s) / (4.3 x 1014 Hz) = 6.98 x 10-7 m = 698 x 10-9 m = 698 nm.
It is preferable to express the wavelength in nm as it is the standard unit used for
wavelengths. Calculate the values for all the wavelengths and enter the values in the
data table on page 5 & 6.
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13. What is the wavelength range for green light?
a. 484 nm to 429 nm
b. 555 nm to 484 nm
c. 429 nm to 405 nm
d. 700 nm to 610 nm
14. What is the wavelength range for ultraviolet rays?
a. 30 nm – 375 nm
b. 4000 nm – 40,000 nm
c. Less than 10 nm
d. More than 500 nm
N. Use the third / bottom line on the chart to mark the wavelengths of the visible colors.
Since the wavelengths range from 400 to 700 nm choose any scale you wish. Remember
to label your chart so it is clear to the person reading it.
15. Scan your chart and save it as a pdf file on your computer. You will upload it using the
Browse button when you are ready to enter your answers on eCampus.
O. Next you will see the different types of spectra. To do this go to
http://astro.unl.edu/classaction/animations/light/spectrum010.html
P. Click on continuous and see the continuous spectrum.
Q. Click on emission and check-mark Hydrogen to see the emission lines.
16. How many emission lines do you see for hydrogen?
a. 2
b. 3
c. 4
d. 5
R. Click on emission, uncheck Hydrogen and check Helium to see its emission lines.
17. How many emission lines do you see for helium?
a. 2
b. 3
c. 4
d. 5
18. Is the red line in hydrogen at the same location as the red line in helium?
a. Yes
b. No
http://astro.unl.edu/classaction/animations/light/spectrum010.html
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19. Which red line has a higher wavelength?
a. Hydrogen
b. Helium
S. Next, change the “spectrum” to “Absorption.” Uncheck Helium and check Hydrogen
again. Finally set the “Spectral Type” to A5 using the slider bar. You will be viewing the
hydrogen absorption spectrum.
20. How many absorption lines do you see?
a. 2
b. 3
c. 4
d. 5
T. See the absorption lines for helium, and keep the slider bar at O or B. Remember to
uncheck hydrogen!
21. What do you notice about the location of the emission and absorption lines?
a. There is no connection between them
b. The wavelengths of the emission and absorption lines are the same
c. There are more emission lines than absorption lines in both cases
d. There are more absorption lines than emission lines
22. Click on the other options to see how the spectra change when you change the
elements. Which element showed you most spectral lines? Why do you think this
happens?
U. Next see images of the REAL spectra on page 12 and answer the questions below. You
are being asked to measure the wavelengths of the different lines in the various
elements. To do this you will use the continuous spectrum with the markings at the top,
as a reference. These markings are in Angstroms, which is just another unit for
wavelength as described on page 1. You can use a ruler to line up the colored spectral
lines with the reference line and read the wavelengths as accurately as possible.
23. Estimate the wavelength of the turquoise blue line in Hydrogen.
24. Estimate the wavelength of the yellow line in helium.
25. Estimate the wavelength of the red line in oxygen.
26. Estimate the wavelength of the most prominent (thickest) green line in nitrogen.
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27. Estimate the wavelength of the most prominent line in sodium.
28. Of the elements shown, which one shows most spectral lines?
a. Hydrogen
b. Nitrogen
c. Sodium
d. Helium
29. Summarize what you learned from this experiment.
Grading rubric:
Pre-lab questions: 0.5 points each = 4 points
Questions 1-6 = 1 point each = 6 points
Question 7 (chart) = 4 points
Questions 8-13: 0.5 points each = 3 points
Questions 14-21: 1 point each = 8 points
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Continuous Spectrum
Hydrogen
Helium
Oxygen
Nitrogen
Sodium
Spectra taken from
http://astro.u-strasbg.fr/~koppen/discharge/
http://astro.u-strasbg.fr/~koppen/discharge/
