The Electromagnetic Spectrum
Introduction
Electromagnetic waves are produced when charged particles are accelerated or change their
energy state. The light you see is a form of electromagnetic radiation, typically produced when
electrons lose energy. The “lost” energy is emitted in the form of electromagnetic radiation.
Radio waves, TV waves, microwaves, X-rays, gamma rays, infrared rays, and ultraviolet rays are
also examples of electromagnetic radiation. Visible light is only a small part of the
electromagnetic spectrum.
In general, electromagnetic waves that are more energetic than visible light (to the right of visible
light in the figure above) are damaging to humans. There are exceptions; microwaves can interact
with water molecules in your tissues to cook you from inside out.
In today’s lab, you will use your eyes as your “radiation detector,” so let’s focus on the visible
spectrum.
ROY G BIV!
“White” is actually what you perceive
when “all” colors are present (or—in
the case of a monitor—red, green, and
blue pixels are all fully illuminated).
“Black” is what you perceive when no
light is present (or—in the case of a
monitor—red, green, and blue pixels
are not illuminated at all)
To study the spectrum of a light source, we need a device that shows all the colors present. You
are familiar with prisms, which separate white light into “rainbow” colors.
For today’s lab, you will use diffraction gratings, which perform the same task.
On the left, you see “magenta” light being “split” into blue and red. On the right, you see the
spectrum of light from the gas inside the glowing “white” tube.
I will provide you with a sheet of diffraction grating, a “spectroscope” which uses a small piece
of diffraction grating, and several light sources. Some of the sources produce light by passing a
high-voltage current through a tube filled with a gas. The current “excites” electrons in the gas
atoms by giving them extra energy. The electrons quickly get “tired” of being “excited,” and emit
light in the process of returning to their normal energy state. Because electrons in atoms exist in
only well-defined energy states, only light corresponding to the energy difference between those
well-defined states is
emitted.
If all that sounds too
complicated, just think of
the spectrum as being the
“fingerprint” of an atom.
Each atom, molecule,
chemical, compound,
etc., has its own unique
fingerprint.
Has anybody been to the sun? How do we know what is in the sun?
One more thing. A continuous spectrum is produced by an object emitting all colors. An
emission spectrum is produced by a glowing gas. An absorption spectrum is produced when
white light passes through a cold gas. If the gas is the same, emission and absorption spectra are
the “inverse” of each other, as pictured above.
As an example, below is the spectrum of xenon gas (rather complex!).
Your lab today involves observing and describing a variety of spectra. I will provide at least one
unknown. To complete today’s lab, complete the observation sheet, identify the unknowns, and
hand in the observation sheet with your name on it. You may work in groups of up to three, so
three names may appear on your observation sheet.
Some web sites to visit:
http://cse.cosm.sc.edu/hses/StarEvol/pages/morelec.htm
http://www.colorado.edu/physics/2000/waves_particles/
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/grating.html
http://amazing-space.stsci.edu/resources/explorations/light/ems-frames.html
http://hyperphysics.phy-astr.gsu.edu/hbase/ems3.html#c2
The Electromagnetic Spectrum Lab Activities
Part 1. Diffraction grating.
I will provide each of you with a sheet of diffraction grating material. I will suggest several activities
that will help you observe and understand its properties.
Part 2. Observations.
Observe each of the light sources with your spectroscope. Sketch the spectrum you observe and state
whether the spectrum appears to be an absorption spectrum, an emission spectrum, or a continuous
spectrum. See the notes below. Warning: if you experience visual problems or a headache while
doing this lab, immediately stop and discuss the problem with your lab instructor. When you get to the
laser, do not look directly at the laser.
Source
Sketch of Spectrum
Emission/Abs/Cont?
Mercury
_____________________________
________________
Sodium
_____________________________
________________
Hydrogen
_____________________________
________________
Helium
_____________________________
________________
Krypton
_____________________________
________________
Argon
_____________________________
________________
Neon
_____________________________
________________
Solar Spectrum1
_____________________________
________________
Fluorescent Tube2
_____________________________
________________
Incandescent Bulb
_____________________________
________________
Computer Monitor3
_____________________________
________________
HeNe Laser4
_____________________________
________________
Unknown
_____________________________
Identify: _______________
Unknown
_____________________________
Identify: _______________
Notes.
1. Attempt to view the solar spectrum. You will probably observe a continuous spectrum because the
thousands of absorption lines in the sun are too narrow to see with your spectroscope. If there are
clouds, you will not be able to observe the solar spectrum with your spectroscopes. Do not look
directly at the sun! Look at a bright reflection.
2. Observe one of the fluorescent tubes either in the laboratory or in the hall.
3. Bring up a screen that is mostly white (e.g., blank document in Microsoft Word), and observe the
white with your spectroscope. Keep in mind that monitors are usually RGB monitors.
4. Lasers are especially useful because their light has a single wavelength and is coherent (all of the
light waves are in phase). Point the laser at the wall and observe the dot on the wall through your
spectroscope. Never look directly at the laser!