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Chapter 24
Electromagnetic
Waves
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
The Nature of Electromagnetic Waves
Benefits of Electromagnetic Waves
1. Visibility (Every thing we see can be attributed to Electromagnetic
Waves)
2. Satellite Communication
3. Telecommunication
4. Remote Control
5. Microwave Oven
6. X-Rays
7. Radio Transmission
8. Laser Surgery
9. Aircraft Navigation.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
The Nature of Electromagnetic Waves
Electromagnetic Wave is a coupled
time varying electric and magnetic fields
that propagate in space.
Two straight wires connected to
the terminals of an AC generator
can create an electromagnetic
wave.
Only the electric wave traveling to the
right is shown here.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
24.1 The Nature of Electromagnetic Waves
Faraday – Lenz stated that TimeVarying Magnetic Field generates
an Electric Field.
Changing Magnetic Flux induces
an EMF which in turn produces a
Current, hence an Electric Field.
Maxwell stated that if a time
varying magnetic field generates an
electric field, then a time varying
electric field should generate a
magnetic field.
The current used to generate
the electric wave creates a
magnetic field.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
The Nature of Electromagnetic Waves
This picture shows the wave of the radiation field far from the
antenna. (Similar to oscillating pendulum. Acceleration of charges
Produces a changing electric field)
Frequency of oscillating charge is frequency of electromagnetic wave.
The speed of an electromagnetic wave in a vacuum is:
c  3.00 10 m s
8
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
The Energy Carried by Electromagnetic Waves
The total energy density carried by an electromagnetic
wave
Total energy 1
1 2
2
u
 oE 
B
Volume
2
2o
Energy in an electromagnetic wave is partly carried by the electric field
and partly by the magnetic field.
o
o
Represents permittivity of free space.
Represents permeability of free.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
The Energy Carried by Electromagnetic Waves
Properties of Electromagnetic Waves
1. Velocity of Electromagnetic Wave in free space or vacuum is a constant.
c
1
 o o

8.85 10
12

1

C 2 N  m 2 4 10 7 T  m A

 3.00 108 m s
2. No material or medium is necessary for propagation.
3. Propagates in the presence of material medium.
4. Electromagnetic waves carry energy and momentum.
5. Electromagnetic waves exert pressure known as Radiation Pressure.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
The Electromagnetic Spectrum
Like all waves, electromagnetic waves have a wavelength and
frequency, related by:
c  f
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
The Electromagnetic Spectrum
Radio
Micro
Infra Red
Visible Light
Ultraviolet
X-Rays
Gamma Ray
Like all waves, electromagnetic waves have a wavelength and
frequency, related by:
c  f
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
The Electromagnetic Spectrum
Maximum Wavelength
Radio
Minimum Frequency
Micro
Infra Red
Visible Light
Ultraviolet
X-Rays
Minimum Wavelength
Gamma Ray
Maximum Frequency
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
The Nature of Electromagnetic Waves
Application of Electromagnetic Waves
1. Radiowaves (produced by accelerated motion of charges in
conducting wires)
Used in radio and television communication.
A radio wave can be detected with a receiving antenna wire
that is parallel to the electric field.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
The Nature of Electromagnetic Waves
With a receiving antenna in the form of a loop, the magnetic
field of a radio wave can be detected.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
The Nature of Electromagnetic Waves
Bands of Radio Waves
AM (Amplitude Modulated): 550 kHz to 1710 kHz
SW (Short Wave): Up to 54 MHz
TV Wave: 54 MHz to 890 Mhz
FM (Frequency Modulated): 88 MHz to 108 MHz
UHF (Ultra High Frequency)
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
The Energy Carried by Electromagnetic Waves
2. Microwaves: Short Wavelength Radio Waves
Produced by Special Vacuum Tubes
Used in Microwave Ovens and RADAR Systems
Microwave is absorbed by water fats and sugars.
RADAR Systems: Air Traffic Control, Speed Detection, etc…
Electromagnetic waves, like water waves, carry energy.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
The Energy Carried by Electromagnetic Waves
3. Infra Red Waves (Heat Waves):
Infra Red Lamps, Infra Red Camera, LED Lamps (Remote Control)
Green House Gas Effect : Sun Rays bounce off earth and get
trapped by CO2, CH4 leading to global warming)
4. Light Waves (Red, Orange, Yellow, Green, Blue, Indigo, Violet)
5. Ultraviolet Waves (Produced by Very Hot Bodies, example Sun).
Ozone Layer protects us from UV Waves.
6. X-Ray (Produced by bombarding metal target by high energy
electrons). Penetrates Flesh Low density but cannot penetrate bone
(high density)
7. Gamma Rays (Produced by Nuclear Reactions and emitted by
Radioactive materials).
Used in cancer treatment.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Polarization
POLARIZED ELECTROMAGNETIC WAVES
Polarization is the process of transforming unpolarized light into polarized light
Polarization is observed only in transverse waves
Unpolarized Wave: Wave vibrates in multiple planes. The vwave vibration
keeps changing but it is always perpendicular to the direction of wave travel.
Polarized wave: Wave vibration is confined to a specific plane.
Polarization is done by scattering, reflection and refraction and also by use of
polaroids.
Polaroids are materials consisting of long chain molecules alighed in a
particular direction.
Intensity of light is lower for polarized light.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Polarization
POLARIZED ELECTROMAGNETIC WAVES
Linearly polarized wave
on a rope.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Polarization
In polarized light, the electric field
fluctuates along a single direction.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Polarization
Polarized light may be produced from unpolarized light with
the aid of polarizing material.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Polarization
MALUS’ LAW
S  S o cos 
2
intensity after
analyzer
intensity before
analyzer
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Polarization
Example 7 Using Polarizers and Analyzers
What value of θ should be used so the average intensity of the polarized
light reaching the photocell is one-tenth the average intensity of the
unpolarized light?
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Polarization
1
10
S o  12 S o cos 2 
1
5
 cos 2 
cos 
1
5
  63.4
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Polarization
When Polaroid sunglasses are
crossed, the intensity of the
transmitted light is reduced to
zero.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.