Transcript Chapter 11: Electromagnetic Waves

Chapter 11:
Electromagnetic Waves
• Section 1: What are Electromagnetic Waves
• Section 2: The Electromagnetic Spectrum
• Section 3: Radio Communication
• Section 4: Quantum Physics (not in text)
Section 1: What are Electromagnetic Waves
What are Electromagnetic Waves (EM waves)?
• Made by vibrating electrical charges
• EM waves travel by transferring energy between vibrating
electric and magnetic fields
• The electrical and fields are perpendicular to each other
• Electric and magnetic fields are created when an electron is in
•
•
•
•
•
motion
An electric current flowing through a wire creates both an
electric fiend and a magnetic field around the wire
The electric fiend and magnetic field are not equal. The electric
field is much stronger than the magnetic field, so it has a greater
effect on matter
As an electron vibrate back and forth the electric and magnetic
fields are changing. A changing electric field creates a changing
magnetic field, which creates a changing electric field. This
results in a continuing EM wave
EM waves have properties similar to other waves—frequency,
wavelength, and amplitude
The speed of any EM wave in the vacuum of space is 3.0 x 108
m/s
Section 2: The Electromagnetic Spectrum
The Electromagnetic Spectrum
• EM waves can have a wide variety of frequency and
wavelength ranges
• The whole range of frequencies and wavelengths is called the
electromagnetic spectrum
• Various portions of the Electromagnetic spectrum interact
with matter differently
• These portions of the spectrum have different names
Radio
Waves
Microwaves
Long wavelength
Low frequency
Low energy
Infrared
Waves
Visible
Light
Ultraviolet
Waves
Xrays
Gamma
Rays
Short wavelength
High frequency
High energy
Relationship between frequency and wavelength
• Remember, the formula for the speed of a wave is: V = f
 The speed (V) of any EM wave is constant
 V = 3.0 x108 m/s
 Also called the speed of light, and symbolized by the letter
“c”
 So, the speed of an EM wave = V = c = 3.0 x108 m/s
 The speed equation becomes: c = f
 “c” is constant, so if the frequency of an EM wave
changes, its wavelength will also change
Section 2: The Electromagnetic Spectrum
Example 1: An EM wave has a length of 7.5 x 10-2-m, what is its
frequency?
Solution
c
m
s
-2
 = 7.5 x 10 m
f=?
c = 3.0 x 108
c = f
c
f
=


c
f=

f=

m
s
f=
-2
7.5 x 10 m
9
f = 2.0 x 10 = 2.0 x 109Hz
s
3.0 x 108
Example 2: An EM wave has a frequency of 2.35x1013Hz, what
is its wavelength?
Solution
c
m
s
f = 2.35x1013Hz
 =?
c = 3.0x108
c =f
c
f
=
f
f
c
=
f
=
f
3.0x108 m
s
=
2.35x1013
s
 = 1.28x10-5m
Section 3: Radio Communication
Radio and Television Communication
1. Radio
• Each radio broadcaster is assigned a specific frequency at
which to transmit
• The EM wave that vibrates at this frequency is the carrier
wave
• The carrier wave is modified on or two ways in order to send
information
 Amplitude modulation (AM) – the amplitude of the wave
is varies
 Frequency modulation (FM) – the frequency of the wave
varied
• A radio detects the variation in the wave’s amplitude or
frequency and converts them into an electronic signal
2. Television
• TV is very similar to radio transmissions
• At a TV station, sound and image are converted to electronic
signals
 The sound is sent by FM signals
 The image information is sent by AM signals
Section 4: Quantum Physics
Quantum Physics
We have been talking about electromagnetism is terms of
waves.
• Two issues that do not fit the wave model
 The wave model cannot account for the spectrum of light
emitted by hot objects (like stars)
 The photoelectric effect – the emission of an electron
from a metal surface when subjected to ultraviolet
radiation.
• In an effort to explain these two issues, physicists developed
the quantum theory
Quantum Physics – a branch of science that deals with discrete,
indivisible units of energy called quanta
Five main ideas in the Quantum Theory:
1. Energy is not continuous, but comes in small, discrete units
2. The elementary particles behave both like particles and like
waves
3. The movement of these particles is random
4. It is physically impossible to know both the position and the
momentum of a particle at the same time. The more
precisely one is known, the less precise is the measure of
the other
• Remember, ρ = mv, so in order to determine the position
of a particle we have to “stop” it, and then we lose any
information about the particle’s momentum. And, as long
as the particle is moving we cannot determine its position
• This is the Heisenberg Uncertainty Principal
5. The atomic world is nothing like the world we know
Section 4: Quantum Physics
• Quanta
 In the late 1890s, the German scientist Maxwell Planck
was doing research on black body radiation
 A black body is an object that is extremely hot and can
only emit radiation through a very small point
 Planck measured the amount of light (energy being
absorbed or emitted by the black body , and determined
that the light is absorbed or emitted in bundles that he
called quanta
 Planck discovered that there was a limit to the amount of
change in energy an atom can experience. That change is
limited to the absorption or emission of one photon
 Through research Planck was able to calculate the energy
of a single photon using the equation: E = hf, where: E =
energy (Joules), f = frequency (/s), and h = Planck’s
constant = 6.626 x 10-34 J٠s
Section 4: Quantum Physics
• Wave/particle duality
One of the most famous experiments in Physics is the Double
Slit experiment. This experiment is important because it
demonstrates the dual nature of light
 The setup is simple: there are two partitions, one with
one slit and the other with two slits. Beyond the second
partition is a photosensitive screen.
Photosensitive Screen
 The experiment is run twice. The
first time the light source can be
a light bulb. In this case, light is
behaving like a wave. Some of
the light passes through the
first partition and continues
onto the second partition. After
passing through the two slits, the light waves
constructively and destructively interfere creating a
pattern of light and dark stripes on the photosensitive
screen.
 This result is not unexpected as constructive interference
would create bright bands where the light is most intense
and destructive interference forms the dark bands (no
light).
Section 4: Quantum Physics
• Wave/particle duality (continued)
 The experiment is run a second time, and the only




difference is the light source is a laser that emits one
photon (particle) of light at a time
The photon passes through the slit in the first partition
and travels to the second partition
Photosensitive Screen
where it passes through both
slits. (Think about it, one particle
passing through two different,
separated, openings at the
Laser
same time)
The photon then travels to the
photosensitive screen.
When enough photons have traveled through the setup
an interference pattern exactly like the one created by the
light bulb forms.
The only way that such an interference pattern can form is
if each photon travels through both slits in the second
partition. Sensors located at both slits in the second
partition detect a single photon at both slits at the same
time. That is very weird!
Section 4: Quantum Physics
Problems using Planck’s Constant
Example 1: What is the energy of the photon when the EM
wave has a frequency of 4.5 x 1014Hz?
Solution
E = hf
f = 4.5 x 1014Hz
14
h = 6.626 x 10-34 J s
E=?
E = (6.626 x 10-34 J s )( 4.5 x 10
E  2.98x10
-19
J
Example 2: If the photons have an energy of 6.0 x 10-20 J,
what is the frequency of the EM wave?
Solution
E = 6.0 x 10-20J
E = hf
h = 6.626 x 10-34 J s
f=?
E
hf
=
h
h
E
f=
h
6.0 x 10-20 J
6.626 x 10-34 J s
9.06 x 1013
f=
s
f = 9.06 x 1013 Hz
f=
s