1. Electromagnetic Waves

7/31/2019 1. Electromagnetic Waves

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SACE Stage 2 Physics

Light and Matter

Electromagnetic Waves


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Characteristics of Electromagnetic Waves

Light is considered to consist of oscillating electric and magnetic fields

at 90o to each other and at right angles to the direction of travel.

Y

Z

X

E

EB

B

Direction of propagation


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Electromagnetic waves are generated by accelerating (vibrate) electrons.

Electrons which vibrate in a single plane will generate electromagnetic waves

with the electric field restricted to a single plane.

Similarly the magnetic field will also be restricted to a plane at right angles to

the electric field. A polarised wave is one in which the electric field is confinedto a single plane and the magnetic field vectors are confined to a single plane

at right angles to the electric field.

We define the plane of polarisation to be the plane of the electric field in

the polarised electromagnetic wave.


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Production of Electro-Magnetic (E-M)

Wavesby an Antenna

When any charged particle (eg. an electron) accelerates, an E-M wave is

produced. If a circuit, therefore, has a vibrating or alternating current in it the

charges are continually accelerating thus will radiate E-M waves.

An alternating voltage applied to a length of metal (an antenna) will thus

radiate an E-M wave.

~ ~E

E

E

E

~

E++

+

--

-

~

+

+

+

--

-

~

+

+

+

--

-

Two metal rods are connected to an AC generator. The charges on each rod then alternate, creating an

alternating Electric field, thus radiating an E-M wave - which travels at the speed of light.


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Reception of an E-M Wave by an

Antenna:

When an EM wave hits an antenna, the free electrons in the antenna will be

forced to vibrate (by induction). If a simple circuit is connected to the antenna

and the circuit is tuned, so that a narrow band of frequency will cause the

electrons to resonate.

This signal is the amplified and sent to the appropriate audio visual device.

The orientation of the antenna should match the plane of polarization of the

EM wave.


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Speed, Frequency and Wavelength

In a vacuum any electromagnetic wave will travel at the speed of light

c= 3 x 108 m s-1. (This is a constant and is the fastest speed possible.)

Since

v

s

t

for a wave, the time taken for one

complete wavelength

( s= ) to pass a point is the period (T)of the wave. Thefrequency of

the wave is the number of waves past a point in one second, hence,

f =1/T


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Thus, v= c, s = , and t= T =1/f

Then, becomes,

Ie,


vs

t

f

f

1T

c l

l

l

c f l


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Example

A ray of green light has a wavelength of540nm in a vacuum. Find its

frequency.


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Example

A ray of green light has a wavelength of540nm in a vacuum. Find its

frequency.

Hzf

Hzf

f

cf

fc

mnm

14

15

7

8

7

1056.5

105556.0

1040.5

103

1040.5540

l

l

l


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Example

A radio station broadcasts at a frequency of720kHz. Find its wavelength.


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Example

A radio station broadcasts at a frequency of720kHz. Find its wavelength.

mm

f

c

fcHzkHzf

4171017.4

1020.7

103

1020.7720

2

5

8

6

l

l

l

l


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Application: LADS

LADS (Laser Airborne Depth Sounder) is a system that uses an aircraft to fly

over a body of water to automatically measure the depth of the water.

The LADS system works by emitting a

laser at the water. When the laser hitsthe water, it is partially reflected and

transmitted. The reflected laser the

returns to the plane. The transmitted

laser continues through and is reflected

of the bottom of the water and back up

to the plane. Knowing the speed of lightin water, the time delay between the

reflected and transmitted ray can be

used to determine the depth.


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Application: LADS

The extra distance travelled by the second pulse (transmitted ray) is to the

bottom of the water and back, i.e., twice the depth of the water. Therefore the

depth of the water is half of the extra distance travelled.


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Application: LADS

Example

An aircraft carrying a LADS system is flying horizontally above a lake. The

laser pulses are reflected of the surface of the water and of the bottom of the

lake return to the aircraft 3.33 s and 3.55 s after being emitted. If the speed

of light in fresh water is 2.25 x 108ms-1, calculate the depth of the water where

the sounding was taken.


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Application: LADS

Example

A aircraft carrying a LADS system is flying horizontally above a lake. The

laser pulses are reflected of the surface of the water and of the bottom of the

lake return to the aircraft 3.33 s and 3.55 s after being emitted. If the speed

of light in fresh water is 2.25 x 108ms-1, calculate the depth of the water where

the sounding was taken.

ms

s

vts

sssst

5.49

102.21025.2

102.222.033.355.3

78

7

mmm

The extra distance travelled by the second pulse is 49.5m, therefore the

depth of the lake is 24.75m deep.


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The Necessity To Use A Powerful Laser

Lasers used in the LADS system are 1Megawatt. This is a very powerful

laser! Why?

Suspended sediment in the water can scatter the laser reducing the amount

of energy reaching the bottom.

Water tends to absorb the light that passes through it.

The seabed also absorbs light.

Only a small fraction of the energy actually return to the aircraft as it scans

from side to side and the surface of the water and seabed is quite rough.

The laser used is very power full and could blind someone below, thereforethe laser is spread out over a distance to reduce the risk of injury below.

1. Electromagnetic Waves

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