Spectroscopy is the study of the interaction
of electromagnetic radiation and matter. What is electromagnetic
radiation? It is any form of radiant energy that is
propagated as waves, and includes what we commonly
call "visible light". Electromagnetic
radiation is characterized by the wave properties of
frequency (ν), wavelength ( λ), and velocity (c).
All electromagnetic radiation travels in a vacuum at
a constant velocity: c = 3.00 x 108 m/s, the speed of
For light waves, the frequency and wavelength are
inversely related, and their product is equal to velocity:
ν λ= c
Frequency is generally measured in Hertz-cycles per
second (written 1/sec or sec-1). Wavelength can be measured in any
unit for length, but nanometers (1 nm=1x10-9m) and meters are common.
The energy (E) of electromagnetic radiation is directly
proportional to its frequency:
E = h ν
Where h is Planck's Constant, equal to 6.626 x 10-34
J-sec/photon. Thus, given any one of the three parameters, frequency,
wavelength, or energy, the other two can be calculated.
You might have noticed that Equation 1 talked about
electromagnetic radiation as a wave, and that Equation 2 referred
to individual particles-photons. Scientists have determined that
it is best to think of electromagnetic radiation as being both wave-like
and particle-like (they call this wave/particle duality). This is
why you will sometimes hear Equation 1 applied to a photon and Equation
two applied to a wave. Both are correct!
The entire range of wavelengths of electromagnetic
radiation is known as the electromagnetic spectrum, shown in Figure
Figure 1: The Electromagnetic Spectrum
As we can see from Figure 1, the electromagnetic
spectrum is made up of many different kinds of radiation, from low
frequency, low energy radiowaves with long wavelengths to high frequency,
high energy gamma rays with short wavelengths. This experiment targets
the visible region of this spectrum specifically, because we can
see electromagnetic radiation with wavelengths in this region with
our own eyes!
Spectra are observed when matter emits or absorbs
light. When an object acts as a source of radiation, it is said
to emit radiation. When radiation from another
source interacts with matter, the matter is said to absorb
Examples of light-emitting objects include the Sun,
a red-hot iron bar, and the hot tungsten filament of an electric
light bulb. Emission of most radiation by matter occurs when its
atoms or molecules are excited by the absorption of energy from
another source. The source of all three examples of emission given
above is the extremely high temperature of the light-emitting object.
An emission spectrometer is used to analyze light
emitted from an excited source. As stated above, when radiation
from an external source interacts with matter, absorption occurs.
Certain characteristic frequencies of radiation are absorbed by
each kind of matter and these frequencies are thus missing from
the spectrum of radiation reflected from that object. A red apple
is absorbing white light and reflecting wavelengths of visible light
that are in the red region. An absorption spectrometer
is used to analyze light reflected by or transmitted through matter.
Why use spectrometers? What do they tell us about
the chemicals or systems under study? Spectroscopy is a fascinating
way to probe the structure and composition of different molecules.
Using spectroscopy, chemists can identify different species present
in a sample or "map out" the structure of a molecule.
One of the earliest clues that changed much of how
we think about elemental substances was borne of spectroscopy. In
1885, Angstrom excited atomic hydrogen and recorded a series of
visible lines that were emitted. The spectrum found is much different
than the "rainbow" spectrum one sees when looking at white
light through some sort of scope. Instead, the spectrum of atomic
hydrogen contains a few narrow bands or lines of specific colors.
Hydrogen is not the only element whose atoms possess
an emission spectrum - the atoms of all elements do! Each has its
own unique emission spectrum that consists only of a few narrow
lines. This is very useful to the chemist; it is an elementary way
of identifying elements! It is among the very best ways to identify
the elements in samples are varied as seawater and distant stars.
And it is exactly what you will employ today to determine the wavelengths
of the hydrogen emission spectrum called the Balmer series. You
will also use emission spectroscopy to identify an unknown cation
in a salt.
It might seem that spectroscopy is only a qualitative
way of identifying different materials, but it is quite handy for
quantitative measurements, too. Modern atomic orbital theory began
with the excitation of an element's atoms by Angstrom, and has matured
today to include the following important points about the hydrogen
- Electrons of an atom have only certain allowed energies, and
each of these energies is associated with a particular atomic
orbital. It is by this description that we can also say that the
energy of electrons is quantized.
- The electron in the hydrogen atom is contained in a particular
- This orbital and thus the electron in it are associated with
a particular energy, E.
- Only certain orbitals, and thus only certain energies, are
allowed. Each of these allowed orbitals is assigned an integer,
n, known as the principle quantum number.
- When light is absorbed, the electron "jumps" from
one allowed energy level (orbital) to another. The energy corresponding
to this "jump" is described as ΔE, or the difference
in energies from the lower state to the higher state. The absorbed
light has a wavelength corresponding to ΔE.
- For hydrogen, this electronic transition energy can be mathematically
described by the Rydberg equation:
E = RH( 1/n2initial -
Where RH=2.18 x 10-18L J/photon, and ninitial
and nfinal are the principle
quantum numbers of the initial and the final energy levels, respectively.
If photon emission occurs, ninitial > nfinal,
and E will be negative. If a
photon is absorbed, ninitial < nfinal,
and E will be positive.
Let's look at an example for clarification. An electron
could be in hydrogen's lowest state (n=1, the ground state). Then,
it absorbs a very specific amount of energy and is elevated to a
higher energy orbital (say, n=3). The Rydberg equation would describe
ΔE as positive, because ninitial=1 and nfinal=3. Then, the atom
emits this energy, as the electron "falls" from
n=3 to n=1. The Rydberg equation gives a negative ΔE
for this transition, as now ninitial=3 and nfinal=1.
This model explains why the frequencies of light
absorbed are exactly equal to the frequencies of light emitted by
a hydrogen sample. Electrons simply move from one energy level to
another, either "up" or "down". Note that this
also explains why emission only occurs after absorption of energy
from another source; electrons must be excited above n=1 for emission
to occur. Use of the Rydberg equation is permissible for all atoms
or ions with one electron. The general equation includes a term,
Z, for atomic number. For hydrogen, Z=1. For all other atoms or
ions, Z is a positive, whole number. The general Rydberg equation
ΔE = RHZ2( 1/n2initial
Today, you will use the Rydberg equation when determining
the spectral lines of the hydrogen emission spectrum. You will also
look at the spectral lines and emission spectra of other elements
Since this experiment focuses on the visible region
of the electromagnetic spectrum, some of the observing you do will
be with your eyes. However, if we wish to study the frequencies
of light emitted by or absorbed from a source, some way must be
found to disperse these frequencies into a spectrum. For example,
when white light is passed through a prism, a spectrum of colors
results. This phenomenon occurs because the different wavelengths
are refracted in the prism to different degrees. Accordingly, the
resultant beam of light is split according to wavelength, or color.
Rainbows are produced in a similar way, with raindrops acting as
An instrument that disperses radiation is variously
called a spectroscope, spectrograph, or a spectrometer.
A simplified diagram of the spectroscope you will
use today and how it works is shown below in Figure 2. The slit
regulates the amount of light which enters the spectroscope and
thus the width of the resulting spectral lines. Light entering the
slit passes through the spectroscope to the diffraction grating
located in the eyepiece. Light passing through this grating is separated
into its components. The scale, which is visible through the eyepiece,
is illuminated by the light entering the spectroscope from a second
direction. Thus, you will see the spectrum superimposed upon the
scale, which is marked off in units of angstroms.
Figure 2: Spectroscope Diagram