Chapter 18 Wave Properties of Light
 18.1 The Electromagnetic Spectrum
 18.2 Interference, Diffraction, and
Polarization
 18.3 Special Relativity
Chapter 18 Objectives

Calculate the frequency or wavelength of light when given one of the
two.

Describe the relationship between frequency, energy, color, and
wavelength.

Identify at least three different waves of the electromagnetic
spectrum and an application of each.

Interpret the interference pattern from a diffraction grating.

Use the concept of polarization to explain what happens as light
passes through two polarizers.

Describe at least two implications of special relativity with regards to
energy, time, mass, or distance.
Chapter 18 Vocabulary Terms
 diffraction grating
 rest energy
 electromagnetic
 special relativity
 spectrum
 spectrometer
 electromagnetic wave
 spectrum
 gamma ray
 time dilation
 inference pattern
 transmission axis
 microwave
 visible light
 polarization
 x-ray
 polarizer
 radio wave
Inv 18.1 The Electromagnetic Spectrum
Investigation Key Question:
What is the electromagnetic
spectrum?
18.1 The Electromagnetic Spectrum
 The energy field created
by electricity and
magnetism can oscillate
and it supports waves
that move.
 These waves are called
electromagnetic waves.
18.1 The Electromagnetic Spectrum
 Electromagnetic waves have
both an electric part and a
magnetic part and the two
parts exchange energy back
and forth.
 A 3-D view of an
electromagnetic wave shows
the electric and magnetic
portions.
 The wavelength and amplitude of the waves are
labeled λ and A, respectively.
18.1 The Electromagnetic Spectrum
 The higher the frequency of the light, the higher the
energy of the wave.
 Since color is related to energy, there is also a direct
relation between color, frequency, and wavelength.
The speed of light waves
 The speed of light is
incredibly fast (3 × 108 m/s)
and is represented by its
own symbol, c.
 The index of refraction (n), is actually the ratio of
the speed of light in a material to the speed of
light in a vacuum.
 The passage of light through matter takes more
time because the light is absorbed and reemitted to pass through neighboring atoms.
18.1 Speed of Light
Speed of light
3 x 108 m/sec
c = f l
Wavelength (m)
Frequency (Hz)
Calculating wavelength
Calculate the wavelength in air of blue-green light that has
a frequency of 600 × 1012 Hz.
1.
You are asked for wavelength.
2.
You are given frequency.
3.
Use speed of light, c = ƒ l
4.
Solve l = c ÷ƒ
 l = (3 x 108 m/s) ÷ ( 600 x 1012 Hz)
 l = 5 x 10 -7 m
18.1 Waves of the electromagnetic
spectrum
 Visible light is a small part of the energy range
of electromagnetic waves.
 The whole range is called the electromagnetic
spectrum and visible light is in the middle of it.
18.1 Waves of the electromagnetic
spectrum
 Radio waves are on the lowfrequency end of the spectrum.
 Microwaves range in length
from approximately 30 cm
(about 12 inches) to about 1
mm.
 The infrared region (IR) of the
electromagnetic spectrum lies
between microwaves and visible
light.
18.1 Medium to high-energy waves
 Ultraviolet radiation has a
range of wavelengths from 400
down to about 10 nm.
 X-rays are high-frequency
waves that have great
penetrating power and are
used extensively in medical
and manufacturing
applications.
 Gamma rays are generated in
nuclear reactions.
Chapter 18 Wave Properties of Light
 18.1 The Electromagnetic Spectrum
 18.2 Interference, Diffraction, and
Polarization
 18.3 Special Relativity
Inv 18.2 Interference, Diffraction, and
Polarization
Investigation Key Question:
What are some ways light
behaves like a wave?
18.2 Interference, Diffraction, and
Polarization
 In 1807, Thomas Young
(1773-1829) did the most
convincing experiment
demonstrating that light is
a wave.
 A beam of light fell on a
pair of parallel, very thin
slits in a piece of metal.
 A pattern of alternating
bright and dark bands
 After passing through the
formed is called an
slits, the light fell on a
interference pattern.
screen.
18.2 Interference
 An interference pattern is created by the
addition of two waves.
18.2 Diffraction gratings
 A diffraction grating is a precise array of tiny
engraved lines, each of which allows light
through.
 The spectrum produced is a mixture of many
different wavelengths of light.
18.2 How a Diffraction Grating Works
When you look at a
diffracted light you see:
 the light straight ahead as
if the grating were
transparent.
 a "central bright spot".
 the interference of all other
light waves from many
different grooves produces
a scattered pattern called a
spectrum.
18.2 Spectrometer
 A spectrometer is a device
that measures the
wavelength of light.
 A diffraction grating can
be used to make a
spectrometer because the
wavelength of the light at
the first-order bright spot
can be expressed in a
mathematical relationship.
18.2 Grating Formula
Distance between grating lines (m)
Wavelength
of light (nm)
l = d sinq
Angle q
18.2 Polarization
 Polarization is another wave property of light.
 The fact that light shows polarization tells us that light
is a transverse wave.
18.2 Polarization
 The direction of polarization is a vector and can be
resolved into components in two directions.
 A wave that has 45-degree polarization is the addition
of two smaller-amplitude component waves with
horizontal and vertical polarizations.
18.2 Polarization
 A wave with polarization
at 45 degrees can be
represented as the sum of
two waves.
 Each of the component
waves has smaller
amplitude.
18.2 Polarization
 A polarizer is a material that selectively absorbs light
depending on polarization.
 A polarizer re-emits a fraction of incident light
polarized at an angle to the transmission axis.
18.2 Applications of polarization
 Polarizing sunglasses are
used to reduce the glare of
reflected light
 The LCD (liquid crystal diode)
screen on a laptop computer
uses polarized light to make
pictures.
Chapter 18 Wave Properties of Light
 18.1 The Electromagnetic Spectrum
 18.2 Interference, Diffraction, and
Polarization
 18.3 Special Relativity
Inv 18.3 Special Relativity
Investigation Key Question:
What are some of the implications of special
relativity?
18.3 Special Relativity
 The theory of
special relativity
describes what
happens to matter,
energy, time, and
space at speeds
close to the speed
of light.
18.3 Special Relativity
These effects are observed in physics labs:
1. Time moves more slowly for an object in motion than
it does for objects that are not in motion. This is
called time dilation.
2. As objects move faster, their mass increases.
3. The definition of the word “simultaneous” changes.
4. Space itself gets smaller for an observer moving
near the speed of light.
18.3 Speed of light paradox
The theory of special relativity comes
from thinking about light.
 A ball thrown from a moving
train approaches you at the
speed of the ball relative to
the train plus the speed of the
train relative to you.
 The speed of light appears
the same to all observers
independent of their relative
motion.
18.3 Speed of light paradox

If the person on the train were to shine a
flashlight toward you, you would expect the
light to approach you faster.

The light should come toward you at 3 ×
108 m/sec plus the speed of the train.

Michelson and Morley found experimentally
that the light comes toward you at a speed
of 3 × 108 m/sec no matter how fast the
train approaches you!
18.3 Consequences of time dilation

In the early 1970s an experiment was performed by
synchronizing two precise atomic clocks.

One was put on a plane and flown around the world,
the other was left on the ground.

When the flying clock returned home, the clocks
were compared.

The clock on the plane measured less time than the
clock on the ground. The difference agreed precisely
with special relativity.
18.3 Einstein's formula
 This equation tells us that matter and energy
are really two forms of the same thing.
Energy (J)
E = mc2
Speed of light
3.0 x108 m/sec
Mass (kg)
18.3 The equivalence of
energy and mass

If a particle of matter is as rest, it has a total amount
of energy equal to its rest energy.

If work is done to a particle by applying force, the
energy of the particle increases.

At speeds that are far from the speed of light, all the
work done increases the kinetic energy of the
particle.

It would take an infinite amount of work to accelerate
a particle to the speed of light, because at the speed
of light the mass of a particle also becomes infinite.
18.3 The equivalence of
energy and mass

Einstein’s was able to deduce the equivalent of mass
and energy by thinking about the momentum of two
particles moving near the speed of light.

Since the speed of light must be the same for all
observers regardless of their relative motion and
energy and momentum must be conserved, as the
speed of an object gets near the speed of light, the
increase in mass must come from energy.
Calculating equivalence
A nuclear reactor converts 0.7% of the mass of uranium to energy. If the
reactor used 100 kg of uranium in a year, how much energy is released?
One gallon of gasoline releases 1.3 × 108 joules. How many gallons of
gasoline does it take to release the same energy as the uranium?
1.
You are asked for energy and no. of gallons.
2.
You are given mass of uranium, % converted to energy, rate
3.
Use Einstein’s formula: E = mc2
4.
Solve for mass converted to energy: m = (.007) ( 100 kg)= 0.7 kg
5.
Solve for energy released: E = (0.7 kg)( 3 x 108 m/s)2

6.
E = 6.3 x 1016 J
Calculate equivalent using rate: 6.3 x 1016 J ÷ 1.3 x 108 J/gal = 4.8 108
J
18.3 Simultaneity
 When we say that two events are simultaneous, we
mean they happen at the same time.
 Since time is not constant for all observers, whether
two events are simultaneous depends on the relative
motion of the observers.
18.3 Simultaneity
 The two lightning strikes
are simultaneous to the
observer at rest, but the
observer moving with the
train sees the lightning
strike the front of the train
first.
Holography
 A well-made hologram appears to
have depth and perspective as if the
actual three-dimensional scene was
embedded in the picture.
 A true 3D scene looks different when
seen from different angles.
 A hologram duplicates the threedimensional shape of the wave front
that is coming from the real object.