What is the nature and properties of light?

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Light plays a vital role in our daily lives. It is used in compact disc (CD) players, in which a laser reflecting off a CD transforms the returning signal into music. It is used in grocery store checkout lines, where laser beams read bar codes for prices. It is used by laser printers to record images on paper.
1. F U N D AM E N T AL S
Nature and Properties
of Light
Linda J. Vandergriff
Director of Photonics System Engineering
Science Applications International Corporation
McLean, Virginia
Light plays a vital role in our daily lives. It is used in compact disc (CD) players, in which a
laser reflecting off a CD transforms the returning signal into music. It is used in grocery store
checkout lines, where laser beams read bar codes for prices. It is used by laser printers to record
images on paper. It is used in digital cameras that capture our world and allow pictures to be
displayed on the Internet. It is the basis of the technology that allows computers and telephones
to be connected to one another over fiber-optic cables. And light is used in medicine, to produce
images used in hospitals and in lasers that perform eye surgery.
The generation, transport, manipulation, detection, and use of light are at the heart of photonics.
Photonics is a critical part of the future and a growing career field. In 1997 it was a $50 billion
market with a projected growth of 10 to 20 percent over the next decade. Photonics technicians
and engineers must master new concepts, learn new techniques, and develop new skills. To
work in photonics it is necessary to have a basic understanding of the nature of light and its
This module requires a basic understanding of high school algebra, trigonometry, general
scientific nomenclature, the scientific process, units conversions, and basic concepts in
elementary physics and chemistry.
When you finish this module you will be able to:
• Describe the wide variety of opportunities for photonics technicians.
• Define and use basic radiometric units.
• Define the following properties of light:
− Speed
− Frequency
− Wavelength
− Energy
• Describe the dual nature of light, as a continuous wave and a discrete particle (photon),
and give examples of light exhibiting both natures.
• Describe the six properties of electromagnetic waves and give everyday examples.
• Explain the mechanism that causes light to be polarized, explain the use of polarizing
material, and give an example of the use of polarizers.
• Describe Huygens’ principle and the superposition principle.
• Define the terms reflection, refraction, and index of refraction and explain how they are
• Explain diffraction and interference in terms of Huygens’ principle.
• List the three types of emission and identify the material properties that control the
emission type.
• Describe in a short paragraph the electromagnetic spectrum and sketch a diagram of the
key optical regions and uses.
• Give a basic explanation of atoms and molecules and their ability to absorb, store, and
emit quanta of energy.
• Define the primary equations describing the relationships between temperature of,
wavelength of, and energy emitted by a blackbody and a graybody.
• Describe the mechanisms that affect light propagating in a medium and its transmission.
True Life Scenario
Holly works as a photonics assembly technician. The factory where she is
employed creates laser diode assemblies for a variety of commercial uses. As
an assembly technician, she creates and aligns laser diode components and
ensures that the bonded products meet the tight quality standard she and her
company require of all their products.
At the beginning of the shift, Holly gets her
assignment for the production run. She
prepares for entering the clean room where
the work will take place. Then, after entering,
she logs on at her assembly station. She
selects the correct workspace file for the
devices to be manufactured during that shift.
The workspace file selection is based on
wavelength, other optical properties, and
pass/fail criteria set for this particular device.
With the components provided, Holly first
sets up the lateral shearing interferometer
and the microlens/laser diode product. Then
she collimates and directs the laser diode Figure 1-1 Photonics assembly
beam into the beam analysis tool for testing. technician assembling LEDs1
After that she reviews the results on the computer monitor and adjusts the
alignment until the device is acceptable. Finally, she bonds the microlens and
laser diode and stores the final measurements to a log file on the network
server. These measurements, along with those of the other devices created
during this production run, will serve as a statistical base for evaluation of the
production process and the product quality.
Holly will create a batch of microlens/laser diode pairs during her shift. Depend-
ing on the order and application, this run may require days or weeks. Then a
new product will be designed for production, and Holly and her counterparts on
the production floor will create it. This will require an evolving understanding of
light and its uses to allow flexibility in the manufacturing process and keep Holly
and her company competitive in the growing marketplace.
Opening Demonstration
Note: The interactive exercise that follows is to be used as a short introduction to the wide
range of photonics applications. It is intended to stimulate interest in the study of photonics.
Instructions: Create two or three groups. Have each group, with notes and manuals closed and
without repeating, name a use of light technology. When no one in a group can think of a use
that has not already been added to the list, that group drops out. Compare the groups’ lists with
the one following. Did the groups miss any areas? Are there any areas that should be on the
following list but are not?
Source: Laser Focus World, May 1999, 291. Reprinted by permission.
Photonics in Our Daily Lives
Home Store
− Energy-saving fluorescent lamps − Supermarket bar-code scanners
− Infrared remote controls − Credit card holograms
− TV flat panel / large screen
− Optical fibers for cable TV Medical
− Compact disc players − Laser surgery
− IR motion sensors for home security − Medical diagnosis tools
− Video disk players − Microscopes
− Alarm clock radio with LED display
− IR noncontact “ear” thermometers
− Laser welding and cutting
− Infrared remote headphones
− Optical stereo-lithography
Office − Machine vision
− Optical scanners − Image recognition for quality control
− Fax machines − Nondestructive testing
− Optical fiber telephone cables − Precision measurement
− Optical data storage − Optical inspection of labeling and packaging
− Laser printers − Laser fabric cutting machines
− Photocopiers
− Overhead slide projectors
− Laser light shows
− Video teleconferences
− Digital cameras
− Laser pointers
− Night vision goggles
− Computer active matrix displays
− Missile guidance
− Computer displays
− Laser weapons
− Infrared remote connections
− Surveillance cameras
− Special optical computers
− Surveying—alignment and range finders
Car − Computer-generated optical elements
− Infrared security systems − Art gallery holography exhibits
− Optical monitors for antilock brakes
− Optical fiber dashboard displays
− LED traffic signals
− Laser traffic radar
− Solar-powered emergency services
Basic Concepts
A. Introduction to Photonics
Photonics is defined as the generation, manipulation, transport, detection, and use of light
information and energy whose quantum unit is the photon.
Photonics is based on the science of optics and electronics. The origins of optical technology
(photonics) date back to the remote past. Exodus 38:8 (ca 1200 BCE) tells of “the looking
glasses of the women.” In the coming century, photonics promises to be a vital part of the
information revolution.
To enable us to understand and apply photonics, it is necessary to have a basic understanding of
the behavior and properties of light. This course focuses on these fundamentals of photonics and
prepares you for an exciting future as a photonics technician.
B. Photonics Opportunities
There are ten broad areas of employment that are likely to need increasing numbers of photonics
• Medicine-biomedical • Aerospace
• Environmental • Computers
• Energy • Manufacturing with photonics and test
• Transportation and analysis
• Defense • Communication and information
• Public safety
Medicine has seen significant growth in the use of photonics devices in laser surgery and in
noninvasive diagnostic tools. This growth translates into ever-growing opportunities for
biomedical photonics technicians.
On the environmental front, photonics devices can measure the pollutants in our air and water
remotely. Photonics devices can harness renewable energy from the sun, augment other energy
sources, and preserve our natural resources. Transportation will be undergoing significant
changes, such as the introduction of the Intelligent Vehicle Highway System, which provides
guidance, collision avoidance, and continuous tuning of engines based on driving conditions.
The defense industry and public safety agencies need the ability to see and understand the
environment, whether it is an enemy’s movement or a tornado’s path. The defense industry, in
addition, has identified several photonics devices that can neutralize enemy weapons. These
fields will grow and will provide significant potential for technicians who work in the areas of
remote sensing, image processing, and high-energy laser operation.
In the Information Age, photonics will be essential in gathering, manipulating, storing, routing,
and displaying information. New optical computers are proposed for some functions, and
charged coupled devices (CCD) cameras digitize artwork. Compact discs, digital video devices
(DVD), and other media are used for data storage and retrieval using lasers. The links between
nodes of the Internet or telephone lines make use of fiber optics. Data can be printed with laser
printers or displayed on plasma panels. This area of photonics application is growing at an
incredible rate, and the potential for technicians who work in this area is very high.
Automation of manufacturing relies heavily on photonics. Fabrication is performed mainly by
industrial lasers that cut, weld, trim, drill holes, and heat-treat products. To ensure product
quality, inspection is performed using spectroscopy, interferometry, machine vision, and image
processing. As manufacturing becomes more sophisticated in its use of photonics, the demand
for skilled photonics technicians is expected to grow explosively.
C. Properties of Light
What is light? This question has been debated for many centuries. The sun radiates light,
electric lights brighten our darkness, and many other uses of light impact our lives daily. The
answer, in short, is light is a special kind of electromagnetic energy.
The speed of light, although quite fast, is not infinite. The speed of light in a vacuum is
expressed as c = 2.99 × 108 m/s. Light travels in a vacuum at a constant speed, and this speed is
considered a universal constant. It is important to note that speed changes for light traveling
through nonvacuum media such as air (0.03% slower) or glass (30.0% slower).
For most purposes, we may represent light in terms of its magnitude and direction. In a vacuum,
light will travel in a straight line at fixed speed, carrying energy from one place to another. Two
key properties of light interacting with a medium are:
1. It can be deflected upon passing from one medium to another (refraction).
2. It can be bounced off a surface (reflection).
The aspects of light interaction with media other than a vacuum will be addressed further in
Modules 1.3 and 1.4, which deal with geometrical and physical optics, respectively.
The field of detection and measurement of light energy is called radiometry. It uses a
standardized system for characterizing radiant energy. Table 1-1 defines the standard terms used
in this course.
Table 1-1: Radiometric Definition and Units
Term Definition Symbol Units
Quantity Radiant energy Q Joule (J)
Flux Rate of radiant energy Φ Watt (W); Joule/second (J/s)
Flux density Flux per unit area E Watts per square meter (W/m2)
Intensity Flux per solid angle I Watts per steradian (W/sr)
Radiance Flux per unit area per unit L Watts per square meter per steradian
solid angle (W/m2 • sr)
Spectral Radiance per unit Lλ Watts per square meter per steradian
radiance wavelength W
per nanometer
m2 • sr • ∆λ
Dual Nature of Light
Scientists build models of physical processes to help them understand and predict behavior. So
it is with light energy. It is through seeing the effects of light that the models are developed.
Scientists have observed that light energy can behave like a wave as it moves through space, or
it can behave like a discrete particle with a discrete amount of energy (quantum) that can be
absorbed and emitted. As we study and use light, both models are helpful.
Concept of a photon
The particle-like nature of light is modeled with photons. A photon has no mass and no charge.
It is a carrier of electromagnetic energy and interacts with other discrete particles (e.g.,
electrons, atoms, and molecules).
A beam of light is modeled as a stream of photons, each carrying a well-defined energy that is
dependent upon the wavelength of the light. The energy of a given photon can be calculated by:
Photon energy (E) = hc/λ (1-1)
where E is in joules
h = Planck’s constant = 6.625 10–34 J•s
c = Speed of light = 2.998 × 108 m/s
λ = Wavelength of the light in meters
Example 1-1
Photons in a pale blue light have a wavelength of 500 nm. (The symbol nm is defined as a
nanometer = 10 m.) What is the energy of this photon?
E = hc / λ = 6.625 × 10 –34 J • s × 2.998 × 10 8 m / s / 500 × 10 –9 m
6.625 × 10 –34 × 2.998 × 10 8
500 × 10 –9
= 3.97 × 10 –19 J
When ultraviolet light shines on some metal surfaces, it causes electrons to be emitted. This
effect is shown in Figure 1-2. The photoelectric effect did not produce results that matched the
early predictions of wave theory. Two concerns were:
1. More intense radiation (larger-amplitude waves) did not cause emitted electrons to have
more energy.
2. The energy of the emitted electron was dependent on the wavelength of the light, not the
amplitude of the wave.
In the photoelectric effect experiment shown in Figure 1-2, light strikes a metal plate. Electrons
are immediately released. The flow of electricity in the external circuit can be measured and the
number of electrons generated for a given light signal can be determined.
Figure 1-2 Photoelectric effect experiment
If light were a continuous wave, it might wash over the metal surface and interact with the
electrons to give them the needed energy to escape at lower light levels (intensities), but only
after long delays. However, faint light at high frequencies (short wavelengths) caused the
immediate release of electrons. Thus, light knocked the electrons out of the metal surface as if
the light were made of particles—photons.
There is a minimum energy threshold for an electron to escape from the metal. Photons with
frequencies below a given threshold eject no electrons, no matter how intense the light. Photons
with frequencies above the threshold do eject electrons, no matter how low the intensity. The
energy of the released electrons can be calculated from Equation 1-2:
Ee– = hc/λ – p (1-2)
where: p = characteristic escape energy for the metal
Ee– = the kinetic energy of an escaping electron
hc/λ = the energy of the photon of wavelength λ
Example 1-2
We can calculate the threshold wavelength of light needed to just release electrons from gold. This
corresponds to Ee– equal to zero. Solve Equation 1-2 for λ.
Let hc/λ = p, so that
hc/p = λ
The escape energy for gold is pgold = 7.68 × 10–19 J
6.625 × 10 –34 J • s × 2.998 × 10 8 m / s
7.68 × 10 –19 J
= 2.59 × 10 –7 m or 0.259 µm
The photon model, although quite useful in explaining some properties of light, is still closely
related to the wave model discussed below.
Wave Model
The particle-like model of light describes large-scale effects such as light passing through lenses
or bouncing off mirrors (dealt with in Module 1-3, Basic Geometrical Optics). However, a
wavelike model must be used to describe fine-scale effects such as interference and diffraction
that occur when light passes through small openings or by sharp edges (dealt with in
Module 1-4, Basic Physical Optics). The propagation of light or electromagnetic energy through
space can be described in terms of a traveling wave motion. The wave moves energy—without
moving mass—from one place to another at a speed independent of its intensity or wavelength.
This wave nature of light is the basis of physical optics and describes the interaction of light
with media. Many of these processes require calculus and quantum theory to describe them
rigorously. For this text it is sufficient to provide the resulting equations and models to be used
by the photonics technician in real applications.
Characteristics of light waves
To understand light waves, it is important to understand basic wave motion itself. Water waves
are sequences of crests (high points) and troughs (low points) that “move” along the surface of
the water. When ocean waves roll in toward the beach, the line of crests and troughs is seen as
profiles parallel to the beach. An electromagnetic wave is made of an electric field and a
magnetic field that alternately get weaker and stronger. The directions of the fields are at right
angles to the direction the wave is moving, just as the motion of the water is up and down while
a water wave moves horizontally. Figure 1-3 is a one-dimensional representation of the electric
Figure1-3 One-dimensional representation of the electromagnetic wave
The maximum value of the wave displacement is called the amplitude (A) of the wave. The
cycle starts at zero and repeats after a distance. This distance is called the wavelength (λ). Light
can have different wavelengths, such as the blue light and red light shown in Figure 1-3. The
inverse of the wavelength (1/λ) is the wave number (ν), which is expressed in cm–1. The wave
propagates at a wave speed (v). This wave speed in a vacuum is equal to c, and is less than c in
a medium. At a stationary point along the wave, the wave passes by in a repeating cycle. The
time to complete one cycle is called the cycle time or period (τ) and can be calculated using
Equation 1-3.
τ = λ/v (1-3)
Another important measure of a wave is its frequency (f). It is measured as the number of
waves that pass a given point in one second. The unit for frequency is cycles per second, also
called hertz (Hz). As you can see, the frequency and the period are reciprocals of one another. If
the wave speed and wavelength are known, the frequency can be calculated with Equation 1-4.
f = 1/τ = v/λ (1-4)
Example 1-3
For blue light in a vacuum, we can calculate the cycle time and frequency. From a previous
example, we know that the wavelength of blue light is 500 nm and the velocity of light in a vacuum
is c. Plugging in the numbers in Equation 1-3 we get:
500 × 10 –9 m
τ = λ/v = = 1.667 × 10–15 s
2.998 × 10 –8 m / s
Then we can calculate the frequency using Equation 1-4.
f = 1/τ = 1/1.667 × 10–15 s = 5.996 × 1014 Hz
It is possible for a wave to have other than sinusoidal shapes; however, the important concept to
remember is that light waves are transverse electric and magnetic fields changing in space and
time and propagating at the speed of light in a given medium, as we show below.
Concept of light waves—Oscillating electric and magnetic fields
Light waves are complex. They are not one-dimensional waves but rather are composed of
mutually perpendicular electric and magnetic fields with wave motion at right angles to both
fields, as illustrated in Figure 1-4. The wave carries light energy with it. The amount of energy
that flows per second across a unit area perpendicular to the direction of travel is called the
irradiance (flux density) of the wave.
Figure 1-4 Electric and magnetic fields in a light wave
Electromagnetic waves share six properties with all forms of wave motion:
• Polarization
• Superposition
• Reflection
• Refraction
• Diffraction
• Interference
Up to this point we have discussed the direction of light’s propagation and its associated electric
and magnetic fields. Polarization arises from the direction of the E-field vector with respect to
the direction of the light’s propagation. Since a light wave’s electric field vibrates in a direction
perpendicular to its propagation motion, it is called a transverse wave and is polarizable.
A sound wave, by contrast, vibrates back and forth along its propagation direction and thus is
not polarizable.
Light is unpolarized if it is composed of vibrations in many different directions, with no
preferred orientation. See Figure 1-5(a). Many light sources (e.g., incandescent bulbs, arc lamps,
the sun) produce unpolarized light. Vertically polarized light is shown in Figure 1-5(b) and
horizontally polarized light in Figure 1-5(c). Each is an example of linearly polarized light.
Figure 1-5(d) shows linearly polarized light making an angle of θ with the vertical. In this case,
the tilted E-vector can be described by its components, Ex and Ey.
(a) Random vibrations of unpolarized light (b) Linearly polarized in a vertical direction
(c) Linearly polarized in a horizontal direction (d) Linearly polarized in a direction making an angle θ with
the vertical
Figure 1-5 Unpolarized and linearly polarized light
When it happens, as in some cases, that Ex and Ey are not in the same phase—that is, they do not
reach their maxima and minima at the same time—the E-field does not remain oriented in a
fixed, linear direction. Rather, the amplitude maxima of the two components do not occur at the
same time and so-called elliptically polarized light is exhibited. This means that, over time, light
exhibits differing polarization orientations. A special case of elliptical polarization—called
circular polarization—occurs when Ex equals Ey and they are out of phase by 90°.
Certain materials will transmit only selected polarizations. They are called polarizers—or
analyzers—and have many uses. With randomly polarized light, a polarizer will pass light of
one polarization and absorb or reflect other polarizations. A common example of the use of
polarization in our daily life is found in polarizing sunglasses. The material in the lenses passes
light whose electric field vibrations are perpendicular to certain molecular alignments and
absorbs light whose electric field vibrations are parallel to the molecular alignments. The major
component of light reflecting from a surface, such as a lake or car hood, is horizontally
polarized, parallel to the surface. Thus, polarization in sunglasses, with the transmission axis in
a vertical direction, rejects horizontally polarized light and therefore reduces glare. However, if
you consider a sunbather lying on his or her side, wearing such sunglasses, the usual vertical
polarization (transmission axis) will now be at 90° and parallel to the surface and will therefore
pass the horizontally polarized light reflected off the water or the land.
The intensity of light passing through a linear polarizer can be calculated using Equation 1-5.
I(θ) = I0 cos2 (θ) (1-5)
where I(θ) is the light intensity passed by the polarizer
I0 is the incident light intensity.
The angle of the E-field with respect to the transmission axis is defined as θ.
Example 1-4
(a) Given horizontally polarized light, what would be the ratio of the light intensity output to the
light intensity input for θ = 0°, 45°, and 90°?
Solution: Use Equation 1-5 to solve for I(θ)/I0 and plug in the numbers.
I(θ)/I0 = cos (θ)
I(0)/I0 = cos (0) = 1
I(45)/I0 = cos (45) = 0.5
I(90)/I0 = cos (90) = 0
(b) Given two polarizers and incident vertically polarized light, what is the ratio of the resultant
light intensity to the incident light intensity if the polarizers’ transmission axes are both vertical and
parallel? What is the ratio if the axes are crossed, that is, one vertical and one horizontal?
Solution: First, for the parallel polarizers, calculate the I(θ)/I0 for the first polarizer assuming θ is
0. Then take the ratio of the two and repeat for the second polarizer. The resulting ratio is 1. Now,
for the perpendicular polarizers, calculate I(θ)/I0 for the first polarizer, assuming θ is 0. Then take
the ratio of the two and repeat for the second polarizer, this time assuming that θ is 90. The resulting
value is 0, as should be expected from crossed polarizers.
Huygens’ Principle
In the seventeenth century, Christian Huygens proposed a principle that can be used to predict
where a given wave front will be at any time in the future if you know the current location. His
principle assumes that each point along a wave front can be considered a point source for
production of secondary spherical wavelets. After a period of time, the new position of the wave
front will be the surface tangent to these secondary wavelets. Huygens’ principle is illustrated in
Figure 1-6, for five point sources on a wave front.
Figure 1-6 Using Huygens’ principle to establish new wave fronts
For many kinds of waves, including electromagnetic, two or more waves can traverse the same
space at the same time independently of one another. This means that the electric field at any
point in space is simply the vector sum of the electric fields that the individual waves alone
produce at the point. This is the superposition principle. Both the electric and magnetic fields of
an electromagnetic wave satisfy the superposition principle. Thus, given multiple waves, the
field at any given point can be calculated by summing each of the individual wave vectors.
When two or more waves are superimposed, the resulting physical effect is called interference.
Suppose two waves, y1 and y2, have nearly the same wavelength and phase (i.e., the maxima
occur at nearly the same time and place). Superposition of these waves results in a wave (y1 +
y2) of almost twice the amplitude of the individual waves. See Figure 1-7a. This is called
constructive interference. If the maximum of one wave is near the minimum of the other wave,
the resultant (y1 + y2) has almost no amplitude, as shown in Figure 1-7b. This is called
destructive interference.
(a) Mostly constructive interference
(b) Mostly destructive interference
Figure 1-7 Using the principle of superposition to add individual waves
When a ray of light reflects off a surface (such as a mirror), its new direction depends on only
the angle of incidence. The law of reflection states that the angle of incidence on a reflecting
basic surface is equal to the angle of reflection. This is discussed in further detail in Module 1-3,
Basic Geometrical Optics.
Law of reflection: Angle of incidence = Angle of reflection
When a ray of light passes from one medium to another, it changes direction (bends) at the
interface because of the difference in speed of the wave in the media. The ratio of this speed
difference is called the index of refraction (n). The ratio of the indices of refraction and the
direction of the two rays of light for the two media are expressed in Snell’s law as shown in
Figure 1-8 and Equation 1-6.
n2 sin θ (1-6)
n1 sin φ
where n1 and n2 are the indices of refraction for the two media
θ is the angle of incidence
φ is the angle of refraction.
Figure 1-8 Refraction and Snell’s law
Conclusive evidence of the correctness of a wave model came with the explanation of observed
diffraction and interference. When light passes an obstacle, the shadow is not precise and sharp
as geometrical ray theory would predict, but rather diffracted a little into the dark region behind
the obstacle, thus giving the shadow a fuzzy edge. This property of light that causes it to spread
out as it travels by sharp edges or through tiny holes can be explained by light having wavelike
properties. Diffraction is predicted from Huygens’ principle. In Figure 1-9, a wave is incident
on a barrier from the left. The barrier has a slit. Every point on the incident wave front that
arrives at the slit can be viewed as the site of an expanding spherical wavelet. For apertures that
are small compared to the wavelength, the aperture becomes like a source and spherical waves
result. As the slit width d increases, the diffracted wave becomes more and more like the
incident plane wave except for the edges at the shadow.
Figure 1-9 Diffraction of waves through slits of differing size
The first definitive demonstration of the wavelike nature of light was the classical two-slit
experiment performed by Thomas Young in 1801. The two slits are very small compared to
their separation distance. Thus, each slit produces diffracted spherical waves that overlap as
they expand into the space to the right of the barrier. When they overlap, they interfere with
each other, producing regions of mutually reinforcing waves. These appear on the screen as
regions of maximum intensity. Between adjacent maxima is a region of minimum intensity. See
Figure 1-10. The resulting pattern on the screen shows where constructive interference occurs
(maxima, labeled B) and where destructive interference occurs (minima, labeled D). The
experimental layout shown in Figure 1-10 can be used in practice to measure the wavelength of
light. This experiment is covered with more rigor in Module 1-4, Basic Physical Optics.
Figure 1-10 Classic double-slit experiment
The Electromagnetic Spectrum
All electromagnetic radiation has similar wavelike properties differing only in wavelength.
Electromagnetic waves range in wavelength from very long (e.g., electric power line radiation
at 60 Hz) to very short (e.g., gamma ray radiation). This entire range is called the
electromagnetic spectrum. The spectrum shown in Figure 1-11 is divided by the practical
applications for given ranges of frequencies that are set through convention by the sources and
detection devices.
Of primary interest to photonics is the region from infrared to ultraviolet. However, each regime
has some utility. Rotating generators and power lines generate low-frequency waves. These
wavelengths are on the order of 105 to 108 meters. Heinrich Hertz produced radio waves in a
very useful region of wavelengths ranging from 0.3 to 105 meters. Television and radio
broadcasting bands are found in lower wavelengths. The microwave regime ranges from 0.01 to
0.3 meter and provides the radar and satellite communication bands. The infrared region, from 1
µm to 30 µm, was first detected by Sir William Herschel in 1800. This region is subdivided into
five regions: very near (1–3 µm), near (3–5 µm), mid (5–6 µm), far (6–15 µm), and very long
(15–30 µm) infrared. Just as the ear cannot hear above or below certain frequencies, the human
eye cannot detect light outside a small range of wavelengths (0.76–0.49 µm). The ultraviolet
region is a higher-energy region discovered by Johann Ritter. It triggers many chemical
reactions and is what ionizes the upper atmosphere, creating the ionosphere. Wilhelm Röntgen
discovered the X-ray regime in 1895. Its wavelength ranges from 10–8 to 10–11 meters. With its
high energy, it can penetrate flesh and provide an image of higher-density material such as
bones. Gamma rays represent the smallest wavelength (less than 10–13 meter). They exhibit
particle-like properties with great energy and are emitted by the sun, linear and particle beam
accelerators, and nuclear processes.
Figure 1-11 Electromagnetic spectrum
White light is a mixture of light of different colors. Each of these colors has a different
wavelength and, when passed through a transparent medium, refracts differently. Thus, a prism
can separate white light into its component colors, as shown in Figure 1-12.
Figure 1-12 Separation of light into component colors
The colors displayed in visible light are categorized by wavelength. Table 1-2 gives the
wavelengths of these colors. An arrangement showing the different components of light, with
the wavelengths of the components in order, is called the spectrum of the light.
Table 1-2: Visible Spectrum Wavelengths
Wavelength Representative
Color Band (µm) wavelength (µm)
Extreme violet 0.39–0.41 0.40
Violet 0.39–0.45 0.43
Dark blue 0.45–0.48 0.47
Light blue 0.48–0.50 0.49
Green 0.50–0.55 0.53
Yellow-green 0.55–0.57 0.56
Yellow 0.57–0.58 0.58
Orange 0.58–0.62 0.60
Red 0.62–0.70 0.64
Deep red 0.70–0.76 0.72
Spectra of Light Sources
The sources of electromagnetic radiation are many and varied. Usually sources are divided into
two categories, natural and man-made. Examples of natural sources of radiation are the sun,
observable stars, radio stars, lightning, and, in fact, any body that exists at a temperature over
absolute zero. Some of the man-made sources of radiation are incandescent and fluorescent
lights, heaters, lasers, masers, radio and television antennas, radars, and X-ray tubes.
Two types of spectra are important in photonics: the emission and the absorption spectra. An
emission spectrum is from light emitted by a source. An absorption spectrum is from light that
has passed through an absorbing medium.
All materials with temperatures above absolute zero emit electromagnetic radiation. Every atom
and molecule has its own characteristic set of spectral lines. The understanding of the
wavelength and energy that produce the spectral “fingerprint” is built on an explanation of the
atomic and molecular structure. The line spectra observed early in the scientific age led to
significant understanding of the nature of atoms. They even led to the development of modern
quantum theory, which says that light emitted by an atom or molecule has a discrete
wavelength, corresponding to a specific energy-level change within the atom or molecule.
These fingerprints can have any combination of spectral lines, bands, and continuums. Atoms
changing states produce visible and ultraviolet radiation. Molecules changing vibrational and
rotational states produce infrared radiation. For dense materials, many energy states are
available; thus emission and absorption bands cover broad regimes for solids and liquids. For a
less dense gas, the spectral bands are much narrower.
To observe a line or band spectrum, a light is passed through a slit. The image of this slit is then
refracted by a prism or diffracted by a grating, based on the constituent wavelengths of the light.
This is recorded on film or a spectrograph. The lines relate back to the atomic structure and the
unique energy-level changes. Spectroscopy is the science that analyzes line spectra and
identifies constituents of materials.
When atoms are close to each other, their electrons interact and the energy levels split. In a
solid, there are so many levels that a continuous range of frequencies can be emitted or
absorbed. Hot, dense materials emit continuous spectra containing bands of frequencies.
Atomic Structure
All matter is made up of atoms. An atom is the smallest unit that retains the characteristics of a
chemical element. It consists of a positive nucleus surrounded by negative electrons arranged in
distinct energy shells designated K through O, as shown in Figure 1-13. The notation K(2)
indicates that the K-shell is complete when it has 2 electrons. Similarly, L(8) indicates that
8 electrons complete the L-shell, and M(18) indicates that 18 electrons complete the M-shell.
Figure 1-13 Atomic model
We model the energy of an atom with the electrons. When all the electrons are in an unexcited,
or ground, state, the atom is assumed to be at its lowest energy level. When the atom absorbs
energy, electrons can be “excited” and moved into higher-energy shells. As electrons move
from one shell to another, unique amounts, or quanta, of energy are absorbed or emitted. This is
how an atom can absorb or emit light. The light’s unique energy quanta are dependent on the
electronic structure of the atom.
Figure 1-14 Energy-level diagram for a hydrogen atom
An atomic energy-level diagram shows the unique electron energies available in a given atom.
An energy-level diagram for hydrogen is shown in Figure 1-14. Hydrogen has only one
electron, and it can exist in only one of the energy levels shown at a time. The lowest level, E1,
is the ground state. Energy must be added to the atom for the electron to move to a higher level.
Note that energy levels range from a value of –13.6 eV (electron volts) for the lowest energy
level (n = 1) to a value of 0 eV for the very highest energy level (n = ∞)—when the electron
breaks free from the atom.
Suppose a hydrogen atom is in an excited state, say, the n = 3 level. The atom can make a
transition to the ground state by emitting a photon. The energy of the photon equals the change
in energy of the atom, as given by Equation 1-7.
E photon = E3 – E1
= –1.51 eV – (–13.6 eV)
= 12.09 eV
The atom can also absorb photons whose energies exactly match differences between electron
energy levels. For example, a hydrogen atom in the ground state can absorb a photon whose
energy is 12.09 eV. The electron in the atom will move from energy level E1 to energy level E3.
Molecular Structure
Molecules in gases or liquids can also absorb electromagnetic radiation. The photon energy
must match a discrete rotational or vibrational energy level of the molecule. In solids,
absorption is more complex, generally resulting from vibrational energy changes.
Blackbody Radiation
The first step toward developing an understanding of blackbody radiation is to describe the
relationships between temperature, wavelength, and energy emitted by an ideal thermal radiator
(blackbody). Based on our everyday observations, we know that bodies at different temperatures
emit radiation (heat energy) of different wavelengths or colors. For example, the wires in a
heater begin to glow red when heated.
Blackbody radiation is the theoretical maximum radiation expected for temperature-related
thermal self-radiation. This radiation can have a peak energy distribution in the infrared, visible,
or ultraviolet region of the electromagnetic spectrum. The hotter the emitter, the more energy
emitted and the shorter the wavelength. An object at room temperature has its peak radiation in
the infrared while the sun has its peak in the visible region.
The equations for calculating radiation based on temperature use the Kelvin temperature scale.
(Be sure to use the Kelvin scale for all calculations.) The conversions between the different
temperature scales are provided in Equations 1-8 and 1-9.
Fahrenheit (F) to Celsius (C) °C = 5/9 (°F – 32) (1-8)
Celsius (C) to Kelvin (K) K = °C + 273.15 (1-9)
Example 1-5
Convert the following Fahrenheit temperatures to degrees Celsius and Kelvin: 212, 100, 32, 0,
–100, –434.
Use Equations 1-8 and 1-9 to complete Table 1-3 with the correct temperatures.
Table 1-3: Temperature Conversion
Fahrenheit (°F) Celsius (°C) Kelvin (K)
Boiling water 212 100 373
100 38 311
Freezing water 32 0 273
0 –18 255
–100 –73 200
Absolute zero –434 –273 0
A waveband is a portion of the electromagnetic spectrum between defined upper and lower
wavelengths. The energy radiated by a blackbody in a given waveband is the sum of all energies
radiated at the wavelengths within the band. The rate of energy radiation is the power radiated. You
can also add the power over all emitted wavelengths to find the total power radiated by a blackbody.
For a blackbody at temperature T, the power radiated per unit surface area of the radiator is given by
the Stefan-Boltzmann law in Equation 1-10.
Ws = σs T 4 watts/m2 (1-10)
where σs = 5.67 × 10–8 watts/m2•K4 (Stefan-Boltzmann constant)
T = Temperature (K)
The power per unit area, Ws, is called the emitted radiant flux density. A graybody is one that
does not emit as a perfect “blackbody” but at a fraction of the theoretical maximum of a
blackbody. The blackbody’s emitted radiant flux density is reduced by a factor called the
emissivity. The emissivity (ε) is dependent on the material emitting and is less than 1. Thus, for
a graybody the emitted radiant flux density is expressed in Equation 1-11.
Ws = ε σs T 4 watts/m2 (1-11)
Example 1-6
Calculate the radiant flux density emitted by a graybody (emissivity = 0.7) at room temperature
First we must convert 82°F to Kelvin. This is 301 K. We then use Equation 1-11 and plug in the
F watts I
Ws = (0.7) 5.67 × 10 –8
m2 K4 K
(301 K) 4 = 325.5 W/m 2
Spectral distribution
The radiation emitted by a blackbody is distributed over wavelength. The quantity Wλ is called
the spectral flux density. It is defined so that Wλ ∆λ is the power radiated per unit area of
surface for wavelengths in the waveband ∆λ (between λ and λ + ∆λ). In 1900, Max Planck
developed a formula that fits experimental measurements of Wλ extremely well. Planck’s
radiation formula is given by Equation 1-12.
C1 1
Wλ = C2
e T
λ –1
where λ = wavelength (m)
T = blackbody temperature (K)
C1 = 2 π c2 h = 3.75 × 10–16 W•m2
C2 = hc/k = 1.44 × 10–4 m•K
c = speed of light = 3.00 × 108 m/s
h = Planck’s constant = 6.626 × 10–34 J•s
k = Boltzmann’s constant = 1.38 × 10–23 J/K
The blackbody spectral flux density from Planck’s formula is plotted in Figure 1-15 for five
blackbody temperatures. The wavelengths are plotted in units of microns. Notice that the axes
are logarithmic.
Figure 1-15 Spectral radiant blackbody flux density distributions at various temperatures
Wien’s displacement law
The spectral distribution for each blackbody temperature has a maximum, or peak, emission
wavelength. This maximum wavelength is related to the blackbody temperature. The
relationship is given by Wien’s displacement law:
λmax T = 2.898 × 10–3 m•K (1-13)
Wien’s displacement law predicts that the peak wavelength decreases in value as the
temperature of the blackbody increases.
Example 1-7
Calculate the apparent blackbody temperature of the sun. If it is observed that the peak spectral
radiant flux density of the sun is near 490 nm, what is its effective blackbody temperature?
By applying Wien’s displacement law and solving for T, we can find the sun’s effective
λ max T = 2.898 × 10 –3 m • K
2.898 × 10 –3 m • K
T= = 5914 K
490 × 10 –9 m
This equation allows the choice of the wavelength most advantageous for detectors given an
expected target temperature.
Interactions of Light with Matter
When light travels through a medium, it interacts with the medium. The important interactions
are absorption and scattering.
Absorption is a transfer of energy from the electromagnetic wave to the atoms or molecules of
the medium. Energy transferred to an atom can excite electrons to higher energy states. Energy
transferred to a molecule can excite vibrations or rotations. The wavelengths of light that can
excite these energy states depend on the energy-level structures and therefore on the types of
atoms and molecules contained in the medium. The spectrum of the light after passing through a
medium appears to have certain wavelengths removed because they have been absorbed. This is
called an absorption spectrum.
Selective absorption is also the basis for objects having color. A red apple is red because it
absorbs the other colors of the visible spectrum and reflects only red light.
Scattering is the redirection of light caused by the light’s interaction with matter. The scattered
electromagnetic radiation may have the same or longer wavelength (lower energy) as the
incident radiation, and it may have a different polarization.
If the dimensions of the scatterer are much smaller than the wavelength of light, like a molecule,
for example, the scatterer can absorb the incident light and quickly reemit the light in a different
direction. If the reemitted light has the same wavelength as the incident light, the process is
called Rayleigh scattering. If the reemitted light has a longer wavelength, the molecule is left in
an excited state, and the process is called Raman scattering. In Raman scattering, secondary
photons of longer wavelength are emitted when the molecule returns to the ground state.
Rayleigh scattering Raman scattering
Figure 1-16 Rayleigh and Raman scattering
Air molecules (O2 and N2) are Rayleigh scatterers of visible light and are more effective at
scattering shorter wavelengths (blue and violet). Can you use this information to explain why,
on a clear day, the sky looks blue?
If the scatterer is similar in size to—or is much larger than—the wavelength of light, matching
energy levels is not important. All wavelengths are equally scattered. This process is called Mie
scattering. Water droplets effectively scatter all wavelengths of visible light in all directions.
Can you use this information to explain the color of a cloud?
Examine basic properties of light such as the following:
• speed
• wavelength
• color spectrum of visible light
• polarization
Laboratory 1.1A—Finding the Speed of Red Light
in Optical-Grade Plastic
The speed of light in a vacuum, c, is exactly 299,792,485 m/s. Current physical theory asserts
that nothing in our universe can have a speed greater than c. When light, or any electromagnetic
wave, moves in any other medium it will have a speed less than c. In general, the speed of light
through a medium depends upon both the medium and the wavelength of the light. The object of
this experiment is to use the definition of index of refraction and Snell’s law to determine the
speed of red light in optical plastic.
Key Definitions and Relationships
1. The index of refraction for any medium, n, is defined as: ni = where vi is the velocity
of light in medium, i.
n1 sin θ 2
2. Snell’s law =
n2 sin θ1
3. The speed of light in vacuum and air is the same to an accuracy of six significant figures,
so we will use nair = 1.00000.
1. Laser, either a red laser pointer or a Class I or II HeNe laser
2. Laboratory stand and clamps to hold laser and gratings
3. Plastic optical block, approximately 8 cm × 6 cm × 2 cm
4. 8½" × 14" white paper
5. Masking tape
6. Meterstick
7. Protractor
1. Tape a legal-size sheet of blank white paper to the tabletop. Tape a laser pointer on the
left side so the beam is directed left to right across the paper.
2. Turn on the laser. Hold a pencil in a vertical position at the right edge of the paper where
the laser beam hits the center of the pencil. Mark the paper with a small dot or dash. See
Move the pencil directly left one or two inches and repeat. Continue until you have five
or six marks extending left to right across the paper. Draw a “best-fit” straight line
through the marks. Label the point at the left end of this line point O. See sketch.
3. Place the plastic block with the large surface down. The left face of the block should
intersect the line on the paper at a 40- to 50-degree angle, and the laser beam should hit
the left face about 1 cm from its lower left corner. Hold the block firmly in place and
draw an outline of the block on the paper. See sketch.
4. Turn on the laser. The beam is refracted by the block and should exit the block through
the right face. Move your pencil along the right edge of the paper until you find the beam.
Mark the location of this exit beam at five or six places on the paper just as you did for
the straight beam in step 2. See sketch.
5. Remove the plastic block. Draw a best-fit straight line through the marks along the exit
beam path. See sketch.
Mark the right end of the line as point D. Extend this line to the left until it intersects the
line marking the lower edge of the plastic block. This is the point where the beam left the
block. Mark this point as point B. Mark the point where the incident beam hit the left face
of the block as point A. Connect points A and B with the line segment AB. See sketch.
Note: The line segment OA describes the path of the laser beam that is incident on the
block. The line segment AB describes the path of the refracted beam through the block.
The segment BD describes the beam’s path after it exits the block.
6. Use the protractor to draw a
line through point A that is
perpendicular to the left face
of the block. Draw another
line through point B that is
perpendicular to the right face
of the block. See sketch.
7. Measure and record the angle θA between the incident beam and the normal line at the
left face of the block (point A). Do the same for the refracted beam at this face. This is
angle θ′A. See sketch.
8. Measure the angle between the normal line and the incident and refracted (exit) beam at
the right face of the block (point B). Label these as θB and θB′. See sketch.
9. Using θA and θA′, use Snell’s law to find the ratio at the left interface between air
n plastic
and plastic.
n plastic
10. Using θB and θB′, use Snell’s law to find the ratio at the right interface between
plastic and air.
11. The index of refraction of air, nair, has a value of 1.00000. Use this and the results of
steps 9 and 10 to find the numerical value of nplastic from your measurements at A and at
B. Average the two values to determine your final estimate of nplastic.
12. Use the definition of index of refraction and the known value of speed c in a vacuum to
calculate the speed of light in the plastic block.
Laboratory 1.1B—Determining the Wavelength
of Red Light
When a beam of light is incident on a diffraction grating, part of the light will pass straight
through. Part of the light is diffracted to paths that diverge at different angles on both sides of
the original path. The angle θ at which the light diverges is related to the wavelength and
spacing of the lines on the grating. The relationship is described by
mλ = d sinθm where λ is the wavelength of the incident light in meters, d is the spacing
between lines on the grating in meters, m is an integer that takes on the
values 0, 1, 2, …., and θm is the diffraction angle for a particular diffraction
order m.
If the diffraction angle θm can be measured for a particular order m and the grating spacing d is
known, the wavelength of the light can be calculated.
1. Laser, either a red laser pointer or a Class I or II HeNe laser
2. Laboratory stand and clamps to hold laser and gratings
3. Transmission grating with 300 to 800 lines/mm
4. 8½" × 14" white paper
5. Masking tape
6. Meterstick
1. Position the laser so the
beam goes straight
down through the
to the grating surface—
and onto the white
paper. There it
produces a center spot
with diffracted spots on
both sides as shown in
the sketch.
2. Measure the vertical distance from the grating to the paper. Record this as L.
3. Measure the distances from the center spot to the first diffracted spots on both sides.
Average these two distances and record the average as ∆x. For these nearest diffracted
spots, m = 1.
F ∆x I
4. Calculate the diffraction angle using θ1 = tan −1
5. Calculate the wavelength of the red laser light using this first-order diffraction angle, θ1
where m = 1. The equation is then
d sin θ1
λ =
Laboratory 1.1C—The Spectrum of Colored Light
1. Flashlight with focusing capability, similar to the Mini-Maglight series
2. Laboratory stand and clamps to hold laser or flashlight and grating
3. Transmission grating with 300 to 800 lines/mm
4. Red, green, blue, purple, yellow, and orange filters
5. 8½" × 14" white paper
6. Masking tape
Part 1. The Spectrum of White Light
1. Mount the flashlight in a clamp on the stand with the beam projected straight down onto a
sheet of white paper.
2. Mount the diffraction grating in another clamp and position it four or five inches below
the flashlight. Focus the light perpendicularly onto the grating surface. See sketch.
3. Move the grating, and
flashlight if necessary, up or
down until you see clearly
both the light transmitted
straight down through the
grating, forming a white spot
on the paper, and the first
order spectrum of colors. See
4. Draw lines through each color you can identify in the spectrum, and label each line with
its color.
Part 2. The Components of Different Colors of Light
5. Hold a red transmission filter between the flashlight and the grating. What is the color of
the center spot where light is transmitted straight through the grating? List all the colors
that you can clearly identify in the diffracted spectrum on either side of the center spot.
6. Replace the red filter with the other filters in this order: green, blue, purple, yellow, and
orange. For each filter list all the colors you can identify in the diffracted spectrum of the
light formed on either side of the center spot.
7. Answer the following questions with complete sentences.
(a) Why are red, green, and blue primary colors?
(b) What colors of light must be combined to make purple light?
(c ) What colors of light must be combined to make yellow light?
(d) How can a color TV produce any color it needs when it has only red, green, and blue
color guns?
Laboratory 1.1D—The Polarization of Light
1. Flashlight with focusing capability, similar to the Mini-Maglight series
2. Laboratory stand and clamps to hold laser or flashlight and grating
3. Two polarizing filters
4. Microscope slide or similar thin, flat glass plate
5. 8½" × 14" white paper
6. Masking tape
Part 1. Polarizers and Analyzers
1. Clamp a flashlight with the beam projected horizontally about five feet above the floor. It
should be arranged so that it is easy to look directly into the light when you are five or six
feet from the flashlight.
Hold one polarizer at arm’s length in front of you and look at the light through the
polarizer. The light you see is now polarized in the preferred direction of the filter.
2. Hold a second polarizing filter (analyzer) with your other hand. Place it between you and
the first filter. Rotate the second filter about the axis of the light beam. Notice the change
in brightness of the light passing through both filters and reaching you.
3. What can you say about the relation between the polarizing direction of the two filters
when the light transmitted has its maximum brightness?
4. What can you say about the relation between the polarizing direction of the two filters
when the light transmitted has its minimum brightness?
Part 2. Polarization by Reflection
5. Clamp the flashlight so the center of the lens is 5 inches above the table and the light
beam is focused on a spot eight inches horizontally from the flashlight. Place a
microscope slide on the table at the position of the focused spot. See sketch.
6. Position yourself in line with the microscope slide and the flashlight. Move until you can
see the reflection of the flashlight from the slide. See sketch.
7. Hold a polarizing filter so you can see the reflection through the filter. Rotate the filter
about the axis of the reflected beam. See sketch. What do you observe about the
brightness of the reflection as you rotate the filter?
8. How does the reflection from the glass affect the properties of the reflected light?
Problem Exercises/Questions
1. Discuss some of opportunities for technicians in the photonics field.
2. Define the following properties of light:
a. Speed
b. Frequency
c. Wavelength
d. Energy
3. Discuss the dual nature of light wave versus photon and give examples of each.
4. Describe in a short paragraph the electromagnetic spectrum with a diagram of the
wavelength regions and typical applications in those regions.
5. An electron in a hydrogen atom is designated to have energy (relative to infinity) with
a. Any value
b. Any positive value
c. Any negative value
d. Only certain isolated values
6. Give the primary equations describing the relationships between temperature,
wavelength, and energy emitted by a blackbody.
7. Address the mechanisms that affect light propagating in a medium and its transmission.
8. Which of the following light sources emits a continuous spectrum?
a. A neon light
b. A glowing coal
c. A mercury vapor lamp
d. Hot, thin interstellar gas
9. Which of the following colors corresponds to the longest wavelength?
a. Blue
b. Violet
c. Red
d. Green
10. List the six properties of wave motion.
Student Project
Create a presentation that educates others at the elementary school level about photonics and the
uses of light.
Bibliography and Resources
Accetta/Schunder. IR/EO System Handbook. ERIM and SPIE Press.
Cobb, Vickie, Joshua Cobb, and Theo Cobb. Light Action—Amazing Experiments with Optics.
Harper Collins Children’s Books, 1993.
Ford, Kenneth. Basic Physics. Walton, Massachusetts: Blaisdell Publishing Co., 1968.
Hecht, Jeff. Optics Light for New Age.
Jenkins and White. Fundamentals of Optics. New York: McGraw-Hill, 1976.
National Photonics Skills Standard for Technicians. Pittsfield, Massachusetts: Laurin
Publishing Company, Inc., 1995.
Seyrafi, Khalil. Electro-Optical Systems Analysis. Los Angeles: Electro-Optical Research
Company, 1985.
Laser As a Tool (Video) and Career Encounters: Optics and Photonics (Video). Washington,
D.C.: Optical Society of America, Ph 202/223-8130, Fax 202/223-1096, E-mail
The Photonics Dictionary. www.laurin.com/DataCenter/Dictionary//CD/index.htm
Metric Prefixes
Prefix Abbreviation Power of Ten Value
tera T 10 thousand billion
giga G 10 billion
mega M 106 million
kilo k 10 thousand
centi c 10 hundredth
milli m 10–3 thousandths
micro µ 10 millionth
nano n 10 billionth
pico p 10–12 thousand billionths