Download electromagnetic waves

INTRODUCTION
•
•
•
•
•
•
An electric current produces a magnetic field, and a
changing magnetic field produces an electric field
Because of such a connection, we refer to the
phenomena of electricity and magnetism together as
electromagnetism
Electric and magnetic fields work together to create
travelling waves called electromagnetic waves
Such waves are responsible for everything from radio
and TV signals, to the visible light we see all around us,
to X-rays that reveal our internal structure
Electromagnetic waves can be produced by (and
detected as) oscillating electric currents (AC) in a wire
or similar conducting element
An antenna is a device designed
to transmit and receive EM
waves
5. Electromagnetic
Waves
1
PRODUCTION OF EM WAVES (1)
•
•
•
•
•
•
Consider a simple circuit where an AC generator of
period T is connected to the centre of an antenna
The antenna is basically a long straight wire with a
break in the middle
At time t = 0, the generator gives the upper part of the
antenna a maximum positive charge, and the lower
segment a maximum negative charge (a)
A positive test charge placed on the x axis at point P
experiences a downward force, hence the electric field
there is also downwards
A short time later, when the charge on the antenna is
reduced (due to smaller amplitude of AC waveform),
the electric field at P also has a smaller magnitude (b)
In (b), the electric field produced at t = 0 has not
vanished, nor been replaced, but has moved further
from the antenna to Q
5. Electromagnetic
Waves
2
PRODUCTION OF EM WAVES (2)
•
•
•
•
•
•
At t = T/4, the antenna is uncharged, and the field
vanishes (c)
In (d) the charges on the antenna segments change
sign, giving rise to an electric field that point upwards
The field vanishes again at t = 3T/4 (f), after which the
field begins to point downwards once more
The net result is a wavelike electric field moving
steadily away from the antenna
The electric field produced by an antenna
connected to an AC generator propagates away
from the antenna, analogous to a wave on a string
moving away from your hand as you shake it up
and down
The wave produced by the electric field is only half of
the EM wave – there is also a magnetic field wave that
is produced and are perpendicular to each other
5. Electromagnetic
Waves
3
DETECTION OF EM WAVES
•
•
•
•
•
•
Suppose an EM wave moves to the right as shown
As the wave continues to move, its electric field exerts
a force on electrons in the antenna that is alternately up
and down, resulting in an alternating current
If the antenna is connected to an LC circuit as shown,
the resulting current can be relatively large if the
resonant frequency (the frequency of a signal at which
the maximum amplitude is produced) of the circuit
matches the frequency of the wave
This is the basic principle behind radio and television
tuners – when you turn the tuner, you are actually
changing the capacitance or inductance of an LC
circuit, and thus changing the resonant frequency
Whenever an electric charge is accelerated it will
radiate an EM wave
The intensity of the radiated EM waves depends on the
orientation relative to the viewer
5. Electromagnetic
Waves
4
PROPAGATION OF EM WAVES
•
•
•
•
•
•
•
•
Sound waves require a medium through which to
propagate
When the air is pumped out of a glass jar containing a
ringing bell, its sound can no longer be heard, but we
can still see the bell is ringing!
Thus light and other types of EM waves can propagate
through a vacuum
All EM waves travel through a vacuum with precisely
the same speed, c = 3.00 × 108 m/s
To quantify, a beam of light could travel around the
world 7 times in a single second
The speed of light is slightly less in air and in denser
materials such as glass or water
Danish astronomer Ole
Romer (1644-1710) was
the first to estimate c
When Earth is at its
greatest distance from
Jupiter, it takes light 16
minutes longer to travel
between them
5. Electromagnetic
Waves
5
THE DOPPLER EFFECT
•
•
•
•
•
•
•
•
The Doppler effect studied for sound waves is also
applicable to EM waves, but with two differences
First, sound waves require a medium to propagate,
whereas light does not
Second, the speed of sound can be different for
different observers, e.g.. an observer approaching a
source of sound measures an increased speed of
sound, whereas an observer detecting sound from a
moving source measures the usual speed of sound
In contrast the speed of EM waves is independent of
the motion of the source and observer
Thus there is just one Doppler effect for EM waves,
which depends only on the relative speed between the
observer and source
For source speeds u that are small compared to c, the
observed frequency f ‘ from a source with frequency f is
given by: f ‘ = f (1± u/c)
u is always positive, and the plus sign applies to a
source approaching the observer and the minus sign is
for a receding source
Example: An FM radio station broadcasts at a
frequency of 88.5MHz. If you drive your car toward the
station at 32.0 m/s, what change in frequency do you
observe?
5. Electromagnetic
Waves
6
EXAMPLE: DOPPLER RADAR
•
•
•
•
The Doppler effect for EM waves is used in applications
such as the radar units used to measure the speed of
cars, and to monitor the weather
In Doppler radar, EM waves are sent out into the
atmosphere and are reflected back to the receiver
The change in frequency in the reflected beam relative
to the outgoing beam provides a way of measuring the
speed of clouds and precipitation that reflected the
beam
A typical Doppler weather radar operates at a
frequency of 2.7GHz. If a wave transmitted by the
weather station reflects from an approaching weather
system moving with a speed of 28m/s, find the
difference in frequency between the outgoing and
returning waves.
5. Electromagnetic
Waves
7
THE EM SPECTRUM
•
•
•
•
•
•
•
•
When white light passes through a prism is spreads out
into a rainbow of colours, with red at one end and violet
at the other
These various colours of light are all EM waves, and
only differ in their frequency and thus their wavelength
The relationship between frequency and wavelength for
any wave with a speed v is v = fλ
For EM waves, c = fλ, where c in a vacuum is constant
Thus as the frequency of an EM wave increases, its
wavelength decreases
Wavelengths are given in units of nanometres (nm)
1nm = 10-9m, and occasionally the angstrom is used
where 1Å = 10-10m
Example: Find the frequency of red light, with a
wavelength of 700.0nm, and violet light with a
wavelength of 400.0nm.
5. Electromagnetic
Waves
8
REGIONS OF EM SPECTRUM (1)
•
•
•
Radio Waves: f ~ 106Hz to 109Hz, λ = 300m to 0.3m
Lowest frequency EM waves of practical importance,
and used for TV and radio signals which are produced
by alternating currents in metal antennas
Radio astronomers use large dish receivers to detect
molecules and accelerated electrons in space
•
•
Microwaves: f ~ 109Hz - 1012Hz, λ = 300mm - 0.3mm
Waves in this frequency range are used to carry long
distance telephone communications, and to cook food
•
Infrared Waves: f ~ 1012Hz - 4.3×1014Hz, λ = 0.3mm 700nm
IR waves can be felt as heat, can’t be seen with eyes
Many creatures have specialised infrared receptors that
allow them to “see” the infra red rays given off a warm
blooded animal, even in total darkness
IR rays are often generated by rotations and vibrations
of molecules
Thus when IR rays are absorbed by an object, its
molecules rotate and vibrate more vigorously, thus
raising the object’s temperature
Remote controls operate on a beam of IR light, but with
low intensity, and thus can’t be felt as heat
•
•
•
•
•
5. Electromagnetic
Waves
9
REGIONS OF EM SPECTRUM (2)
•
•
•
•
•
•
•
•
•
Visible Light: f ~ 4.3×1014Hz – 7.5×1014Hz, λ = 700nm
– 400nm
Represented by the full range of rainbow colours
Each colour is simply an EM wave with a different
frequency
Such waves are produced by electrons changing their
positions within an atom
Ultraviolet Light: f ~ 7.5×1014Hz – 1017Hz, λ = 400nm
to 3nm
Such rays are invisible, but can cause suntans with
moderate exposure
Prolonged exposure can have harmful consequences
such as development of skin cancer
Most UV radiation from the sun is absorbed in the
upper atmosphere by ozone (O3)
A significant reduction in the ozone concentration in the
stratosphere could result in an unwelcome increase of
UV radiation on Earth’s surface
5. Electromagnetic
Waves
10
REGIONS OF EM SPECTRUM (3)
•
•
•
•
•
•
•
X-Rays: f ~ 1017Hz to 1020Hz, λ = 3nm to 0.003nm
The X-rays used in medicine are generated by the rapid
deceleration of high speed electrons projected against
a metal target
These energetic rays are weakly absorbed by the skin
and soft tissue and pass through our bodies quite
freely, except when they encounter bones, teeth or
other dense material
X-rays can cause damage to human tissue, and so it is
desirable to reduce exposure as much as possible
Gamma (γ) Rays: f > 1020Hz, λ < 0.003nm
More energetic than X-rays, produced as neutrons and
protons rearrange themselves within the nucleus of an
atom, or when particles and antiparticles collide and
annihilate each other
Highly destructive to living cells, and are used to kill
cancer cells and micro organisms in food
5. Electromagnetic
Waves
11