E lec t romagne t ic
Waves
How do you think a bee
locates a flower?
When a bee looks at a flower and you
look at the same flower, do both of you
see the same thing?
Do bees see all the same things we see
when they look at a flower?
Some flowers have markings on them that are
invisible to humans. These markings are only seen
in ultraviolet light.
Objectives:
1. Describe the nature of electromagnetic waves.
2. Explain how electromagnetic waves differ from other waves.
3. Explain how electromagnetic waves are produced.
4. Identify the regions of the electromagnetic spectrum.
5. Describe the uses of the electromagnetic waves of different
frequencies.
6. Explain the relationship between wavelength and frequency.
7. Explain three ways by which luminous objects produce light.
8. Describe the particle nature of electromagnetic waves.
TYPES OF WAVES
Waves are classified into different types
according to their natures :
WAVES
Mechanical waves
Transverse waves
Electromagnetic waves
Longitudinal waves
Transverse waves
In 1879, a Scottish scientist named
James Clerk Maxwell developed a
mathematical description of
Faraday’s “fields”.
(13 June 1831 – 5 November 1879)
Maxwell found that as electric and magnetic fields changed,
they traveled together through space as a single wave, what
we call today an electromagnetic wave. Maxwell’s
mathematics even told him how fast that the waves moved.
1
It was Maxwell’s genius to see that light (which had always
been a mystery anyway) must be made of these waving
electric and magnetic fields.
Not only that, but Maxwell could see that there must be
many other kinds of light — light invisible to our eyes and
unknown to science.
German Scientist Heinrich Hertz first discovered
Maxwell’s predicted radio waves in 1887. To capture
those waves, Hertz used a wire loop.
2
Light has been described as a particle, as a wave and even
as a combination of the two.
The current model for light has incorporated aspects
of both particle and wave theories.
Not all light is visible to the human eye.
3,4
Electromagnetic waves carry no mass, but they do carry energy.
Electromagnetic waves are transverse waves and do not require
a medium. This means that electromagnet waves can travel
through a vacuum (no air).
Each electromagnetic wave emits a different level of energy.
5,6,7
Red – electric field
Blue – magnetic field
Electromagnetic waves are vibrations of magnetic and
electric fields positioned at right angles to each other and
to the direction of the motion of the wave.
8
Red – electric field
Blue – magnetic field
As the electric field increases, it induces an increasing
magnetic field at right angles to it, and vice-versa.
The waves, in the absence of any intervening absorbing
medium, propagate indefinitely.
Electromagnetic waves are distinguished by their unique
wavelengths and frequencies.
In visible light, the difference in frequencies and
wavelength account for different colors.
The difference in frequencies and wavelengths also
distinguishes visible light from invisible electromagnetic
radiation, such as X rays.
9,10,11
One way to measure the energy of an electromagnetic wave
is by measuring its frequency.
Frequency refers to the number of vibrations created in a
period of time.
In general, the higher the frequency, or number of waves, the
greater the energy of the radiation.
Wavelength and frequency are inversely related, meaning the
greater the length of the wave, the lower its frequency will be.
If the frequency is high, the wave must be shorter. The
shorter the wave the higher the energy.
12,13,14
In a vacuum, all electromagnetic waves travel at the speed of
light: 3.0 × 108 m/sec.
This quantity is called “the speed of light” but it really refers
to the speed of all electromagnetic waves, not just visible light.
The relationship between frequency, wavelength and speed also
holds true for light waves.
15
Electromagnetic waves exist in an enormous continuous
range of frequencies.
They are organized in the electromagnetic spectrum
according to their wavelength, from the shortest (gamma) to
the longest (radio).
16
Types of electromagnetic waves along
with their wavelengths and frequencies
are shown at the left.
Although specific ranges are shown in
the table, the electromagnetic
spectrum is, in reality continuous with
no sharp division between one kind of
wave and the next.
Radio Waves
Radio waves are the waves
used to transmit information
from the antenna of a
broadcasting station to the
antennae on your radio.
When you think of radio waves, do
not confuse them with the
waves you hear coming out of your
radio. A radio converts
radio waves into sound.
It was discovered that radio waves could bounce off airplanes
and end up right back where they started. This idea came to
be called radar, which stands for radio detection and ranging.
17,18
When radio waves are transmitted, one of two characteristics
can be varied – either the amplitude or the frequency.
The variation in the amplitude or frequency of a wave is called
modulation. The setting on your radio indicates the type of
modulation used to carry the information to your radio.
AM stands for amplitude modulation.
FM stands for frequency modulation.
19,20,21
AM radio waves have longer wavelengths than FM radio waves
have, they can bend around hills and buildings.
The shorter wavelength FM radio waves are blocked by large
objects.
22
Sound transmitted by an AM radio station affects the carrier
wave by changing the amplitude (height) of the carrier
wave, as shown below.
Unfortunately, this type of modulation is subject to static
interference from such things as household appliances —
and especially from lightening storms.
23
FM radio works by changing the frequency of the carrier
wave.
FM is virtually immune to any type of external interference, it
has a greater dynamic range, and it can handle sounds of
higher and lower frequencies.
This is why music, with its much greater frequency range
than the human voice, sounds better on FM radio.
Note that when the carrier wave of
FM radio is modulated with sound
that the distance between the
waves, or the frequency of the
carrier wave, changes.
24,25
What is the primary use for radio waves? Communication
Satellite Communication
Cordless Phones
26
Wireless
Communication
Microwaves are radio waves with the shortest wavelength
and the highest frequency.
Microwaves are used in cooking,
Communication
Radar
27,28
Infrared light lies between the visible and microwave
portions of the electromagnetic spectrum. Infrared light
has a range of wavelengths.
"Near infrared" light is closest in wavelength to visible
light and "far infrared" is closer to the microwave region
of the electromagnetic spectrum.
Since the primary source of infrared radiation is heat or
thermal radiation, any object which has a temperature
radiates in the infrared.
29,30,31
Far infrared waves are thermal. In other words, we experience
this type of infrared radiation every day in the form of heat! The
heat that we feel from sunlight, a fire, a radiator, a heat lamp or
an electric burner.
Shorter, near infrared waves are not hot at all - in fact you
cannot even feel them. These shorter wavelengths are the ones
used by your TV's remote control.
32,33
A thermogram is a thermal image made from an infrared
camera or other infrared device. Devices that make use of
thermograms are able to display outlines of the subjects
based on the amount of infrared radiation (which corresponds
to the amount of heat) emitted by the body.
Applications for thermograms range from medical
diagnostics to search and rescue devices. These are highly
favored devices because of their non-invasive capabilities.
34,35
Night vision devices make use of infrared by either capturing
the naturally emitted infrared radiation from the subjects or by
using infrared light sources that are not as noticeable as
visible light.
Devices like these are mostly used in the military.
36
Isaac Newton showed that
light shining through a prism
will be separated into its
different wavelengths and
will thus show the various
colors of which visible light
is comprised.
The separation of visible
light into its different colors
is known as dispersion.
37
Visible light waves are the only electromagnetic waves we
can see. We see these waves as the colors of the rainbow.
Each color has a different wavelength.
38,39
Bluer light has shorter wavelength, higher frequency, and
more energy.
Redder light has longer wavelength, lower frequency, and
less energy.
When all the waves are seen together, they make white light.
40,41,42
Ultraviolet (UV) light waves are invisible to the human eye.
Scientists have divided the ultraviolet part of the spectrum into
the three regions (Near UV, Far UV and Extreme UV) shown in
the diagram above.
The three regions are distinguished by how energetic the
ultraviolet radiation is, and by the "wavelength" of the
ultraviolet light, which is related to energy.
43
Our Sun emits light at all the different wavelengths in
electromagnetic spectrum, but it is ultraviolet waves that are
responsible for causing our sunburns.
44
Though some ultraviolet waves from the Sun penetrate
Earth's atmosphere, most of them are blocked from entering
by various gases like Ozone.
Some days, more
ultraviolet waves get
through our
atmosphere.
Scientists have
developed a UV index
to help people protect
themselves from
these harmful
ultraviolet waves.
45
Uses of Ultraviolet Light
Astronomers have to put ultraviolet telescopes on
satellites to measure the ultraviolet light from stars and
galaxies - and even closer things like the Sun!
46
Ultraviolet light frequently used in security. For instance,
currency, has invisible symbols on it that light up only in the
presence of UV light. These are difficult for counterfeiters to
copy.
46
Sensitive documents, such as driver's licenses, credit cards
or passports, have invisible symbols on them that light up
only in the presence of UV light.
46
Ultraviolet rays cause skin calls to produce vitamin D, which
your body needs. However too much exposure is harmful.
Ultraviolet light has been implicated in multiple human diseases
from skin cancer to cataracts to immune suppression.
47,48
The Earth's atmosphere is thick enough that virtually no
X-rays are able to penetrate from outer space all the
way to the Earth's surface.
49
X-rays were first observed and documented in 1895 by
Wilhelm Roentgen, a German scientist who found them
quite by accident when experimenting with vacuum
tubes.
50
A week later, he took an X-ray
photograph of his wife's hand which
clearly revealed her wedding ring and
her bones. The photograph electrified
the general public and aroused great
scientific interest in the new form of
radiation.
Roentgen called it "X" to indicate it was
an unknown type of radiation. The
name stuck, although over Roentgen's
objections.
When an X-ray picture of a part of your body is taken, the bones
absorb the rays, but the soft tissues do not.
The picture that results shows the bones as white areas and
the soft tissue as black areas.
51,52
Despite their usefulness in medical diagnosis, X-rays are a
potential health hazard. Exposure to body cells and tissues to
large amounts of X-rays over a lifetime can cause defects in
cells.
Lead absorbs almost all the X-rays that strike it. X-ray
technicians wear lead aprons to protect them from exposure.
53
Strong sources of X-rays have been detected deep in space.
the sources are believed to be certain star formations.
In addition to these star formations, exploding stars are known
to give off most of their energy at the time of explosion in
the form X-rays.
Gamma rays are the electromagnetic waves with the
highest frequency and the shortest wavelength.
Gamma rays have the highest energy of the electromagnetic
spectrum.
Certain radioactive materials and nuclear reactions emit
gamma rays.
54,55,56
Most of the gamma radiation observed on Earth come
from radioactive substances.
Gamma rays are emitted if a nucleus still has excess
energy following radioactive decay and the emission of
other particles.
57,58
Gamma rays have a high penetrating power - it takes a
thick sheet of metal such as lead or concrete to reduce
them significantly.
59
Physicists credit French physicist Henri Becquerel with
discovering gamma radiation. In 1896, he discovered that
uranium minerals could expose a photographic plate through
a heavy opaque paper.
60
Gamma-rays travel to us across vast distances of the
universe, only to be absorbed by the Earth's atmosphere.
61
Doctors use the gamma rays for treatment purposes,
mainly for killing cancer cells, tumors and other malignant
cells in the human body.
A radioactive substance, known as a tracer is put inside the
human body, and its path (trace) inside the body is
followed.
A special gamma camera
uses those rays to build up
a picture.
The patient however, gets
only a small dose of the
radiation and does not
suffer.
62
They are also used for impurity analysis of metallurgical
specimens or to determine the effects of chemicals on
biological systems and plants.
Engineers make use of the gamma rays, to look for cracks
in pipes and aircraft parts, since they can penetrate better
than X-rays
63,64
Gamma rays can also be used for sterilizing medical products.
Umbilical Cotton tape is sterilized by Gamma radiation Sterilization.
65
http://science.hq.nasa.gov/kids/imagers/ems/uv.html
http://www.msnucleus.org/membership/slideshows/c
oloringbookfinal.html
http://www.msnucleus.org/membership/slideshows/light2.html