Transcript Document

CPO Science
Foundations of Physics
Unit 6, Chapter 18
Unit 6: Light and Optics
Chapter 18 Wave Properties of Light
 18.1 The Electromagnetic Spectrum
 18.2 Interference, Diffraction, and
Polarization
 18.3 Special Relativity
Chapter 18 Objectives
1. Calculate the frequency or wavelength of light when
given one of the two.
2. Describe the relationship between frequency, energy,
color, and wavelength.
3. Identify at least three different waves of the
electromagnetic spectrum and an application of each.
4. Interpret the interference pattern from a diffraction
grating.
5. Use the concept of polarization to explain what happens
as light passes through two polarizers.
6. Describe at least two implications of special relativity
with regards to energy, time, mass, or distance.
Chapter 18 Vocabulary Terms
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x-ray
spectrum
microwave
index of refraction
electromagnetic
wave
spectrometer
gamma ray
radio wave
transmission axis
diffraction grating
special relativity
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polarization
polarizer rest
energy destructive
interference
ultraviolet
time dilation
infrared
speed of light
constructive
interference
 visible light
 wavelength
18.1 The Electromagnetic Spectrum
Key Question:
What is the electromagnetic
spectrum?
*Students read Section 18.1
BEFORE Investigation 18.1
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.
18.1 Speed of Light
Speed of light
3 x 108 m/sec
c = f l
Wavelength (m)
Frequency (Hz)
18.1 Calculate wavelength
 Calculate the wavelength
in air of blue-green light
that has a frequency of
600 × 1012 Hz.
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 (or IR) region of
the electromagnetic spectrum
lies between microwaves and
visible light.
18.1 Waves of the electromagnetic
spectrum
— 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.
18.2 Interference, Diffraction, and
Polarization
Key Question:
What are some ways
light behaves like a
wave?
*Students read Section 18.2
AFTER Investigation 18.2
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 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 ~ dw
L
distance
between
2 first order
bright spots
distance
between
screen and glasses
d= 13,500 lines/inch = ? lines/m
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
 Polarization is a vector.
 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 Polarizers
 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.
18.3 Special Relativity
Key Question:
What are some of the
implications of special
relativity?
*Students read Section 18.3
AFTER Investigation 18.3
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
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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.
But 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
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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
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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
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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.
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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.
18.3 Calculate equivalents
 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?
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.
Application: Holography