Electromagnetic Radiation

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Michelson’s method for measuring speed of light

C = 3.0 x 108 m/s

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Electromagnetic force

Electric and magnetic fields are very closely linked together. This relationship was first recognized by Michael Faraday (1791-1867)

Electric and magnetic forces are two aspects of the one force, called the electromagnetic force.

This is believed to be one of the four fundamental forces in nature. The other fundamental forces are gravity, the strong nuclear force and the weak nuclear force.

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A charged particle with constantly changing velocity produces:◦ a constantly changing electric field, which in turn produces◦ a constantly changing magnetic field, which in turn produces◦ a constantly changing electric field.

The electric and magnetic fields continually reinforce each other in this way. They radiate away from their source as electromagnetic waves.

electromagnetic waves

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Accelerating charges produce electromagnetic waves

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Electric & Magnetic Fields propagate each other.

• As the oscillating electric field is being produced, an oscillating magnetic field is also produced.

• The electric and magnetic fields are at right angles to each other.

• The electric and magnetic fields are at right angles to their direction of travel as shown in Figure 2

• Electromagnetic waves are said to be transverse because they oscillate at right angles to their direction of travel.

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Properties of wavesWavelength, λ, is the distance between two adjacent crests or troughs, measured in metres.

Frequency, f, is the number of waves which pass a given point in one second, measured in Hertz (Hz). The frequency of waves is determined by the source, and once the waves leave the source the frequency does not change, although the wavelength and speed may change.

Amplitude, A, is the maximum displacement of the particles, or electric or magnetic fields, from their central position, measured in metres.

Speed of waves, v, is the speed with which the wave crests travel through a medium or space, measured in m/s

Period, T, the time for one complete oscillation. It can also be the time for one complete wave to pass a given point. Period is measured in seconds.

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Points on a wave are said to be in phase if the waves at those points are moving in the same direction and with the same speed. They can be said to be at the same stage of an oscillation.

A and B are in phase. Points C and D are also in phase. Points E and F are half a period apart and are said to be 180°, or λ, out of phase.

Phase of waves

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Types of Electromagnetic waves

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The speed of all electromagnetic waves is the same as the speed of light, i.e. 3.00 x 108 ms-1. The speed of electromagnetic waves is given the symbol c.

The wave equation relates the speed, frequency and wavelength of all waves, including electromagnetic waves.

◦ v = f λ where v = speed (ms-1)

f = frequency (Hz)

λ = wavelength(m)

For electromagnetic waves, v = c, therefore the equation becomes: c = f λ

Relation between speed, frequency and wavelength

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ExampleThe frequency of the microwaves used by a communications satellite is 5x1010 Hz. What is their wavelength?

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When radio and TV waves are transmitted by an antenna, the electrons can only oscillate in the direction of the antenna. This produces waves with electric fields that oscillate in the same direction, i.e. parallel to the antenna.

Waves which oscillate only in the one direction are said to be polarised.

Only transverse waves can be polarised. Longitudinal waves cannot be polarised because they can only oscillate in the direction of travel of the waves.

The polarisation of electromagnetic waves is evidence that they are transverse waves

Polarisation of electromagnetic waves

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Polarisation of emr

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Polarising sheets

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Rope analogy of crossed polarising sheets

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3D Movies using polarising lenses

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When a city and nearby country areas have different TV channels, there is a possibility that reception from one could interfere with reception from the other.

Polarisation is used to help keep their reception separate. City areas may have horizontally orientated transmitting antennae and receivers to send and receive horizontally polarised waves

The country areas would then have their transmitter and receptors orientated vertically to send and receive vertically polarised waves.

City and country reception

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Until the early 1990s, all surveys of Australian coastal waters were carried out by ships using sonar equipment.

However, there are many problems with this type of survey. In some areas, eg. around the Great Barrier Reef, the waters may be too dangerous for ships to sail through them.

Survey by ships is extremely slow - it is estimated that it would take about 100 years to complete the survey of the whole of Australian coastal waters using sonar.

In the late 1980s, a South Australian company, Vision Systems, developed a survey method using laser light and an aeroplane.

This method is called the Laser Airborne Depth System (LADS), and since 1993 has been used by the Royal Australian Navy to carry out the survey of coastal waters.

Laser Airborne Depth System

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LADS

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LADS The laser used in LADS produces electromagnetic

radiation of frequency 2.8x1014 Hz and wavelength 1.06x10-8 m, which is in the infrared region of the electromagnetic spectrum.

The frequency of the radiation can also be doubled producing light of frequency 5.6x1012 Hz and wavelength 5.3x10-7 m, which is green.

The green light penetrates water quite easily to depths of up to 70m, if the water is sufficiently clear.

The infrared radiation cannot penetrate water and is reflected from the surface.

Equipment in the aircraft records the time interval between pulses of radiation being sent out and their return.

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td = t(d+h) - th

th is the time taken by the infrared radiation travelling the distance from the aircraft to the surface of the water and back again, i.e. twice the height of the aircraft.

t(d+h) is the time taken by the green light travelling the distance from the aircraft to the bottom of the sea and back again, i.e. twice the height of the aircraft plus the depth of the water.

td is the distance between the two times and is the time taken for the green light to travel the distance from the surface to the bottom and back again to the surface

LADS