Electromagnetic Waves

Learning Objectivesa) State that all electromagnetic waves are transverse waves that travel with

the same speed in vacuum and state the magnitude of this speed

b) Describe the main components of the electromagnetic spectrum

c) State examples of the use of the following components:i. radiowaves (e.g. radio and television communication)

ii. microwaves (e.g. microwave oven and satellite television)

iii. infra-red (e.g. infra-red remote controllers and intruder alarms)

iv. light (e.g. optical fibres for medical uses and telecommunications)

v. ultra-violet (e.g. sunbeds and sterilisation)

vi. X-rays (e.g. radiological and engineering applications)

vii. gamma rays (e.g. medical treatment)

d) describe the effects of absorbing electromagnetic waves, e.g. heating, ionisation and damage to living cells and tissue

Electromagnetic Spectrum

How to remember EM Waves??

Electromagnetic Waves

Radio WavesMicrowavesInfra-Red

Visible lightUltra-violet

X-raysGamma Rays

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Properties of EM Waves1. They transfer energy from one place to another.

2. They are transverse waves.

3. They can travel through vacuum.

4. They travel through vacuum with the speed of light = 3.0 × 108 m/s.

5. Their speed is given by v = f (frequency) × λ (wavelength)

6. They obey the laws of reflection and refraction.

Relationship between f and λFor EM waves,

v = f × λ = 3.0 × 108 m/s (constant)

If f decreases, λ must increase.

If f increases, λ must decrease.

Relationship between f and λPractice Question

A wave moving along a length of rope has a frequency of f. Given that the speed of the wave is constant, what happens to the wavelength of the wave when the frequency is increased to 2f?

Using the wave equation:

Wavelength of the wave will be halved when the f of the wave doubles.

𝑣=2 𝑓 ×= 𝑓 l?l2

𝑣= 𝑓 l

Relationship between f and energy

• The greater the frequency f, the greater the energy.

• The SHORTER the wavelength the HIGHER the frequency the GREATER the energy possessed by the wave.

• So gamma rays (highest frequency) has the highest energy.

Uses of EM Waves

Uses of EM WavesRadio waves• Radio waves have the longest wavelengths in the

electromagnetic spectrum• Wavelengths range from several hundred metres to a few

centimetres

 Uses: to transmit sound and pictures in radio and television

Uses of EM WavesMicrowaves• Microwaves are radio waves of very short wavelengths (from

10–3 m to 10–1 m)

Uses:Satellite

communication for satellite television, GPS and mobile

Microwave ovens Radar Communication

Uses of EM WavesInfrared-red radiation• These are waves just beyond the red end of the visible light

spectrum• Wavelengths range from 10–7 m to 10–3 m• All objects emit infra-red radiation

Uses:Remote control

of electrical appliances

Intruder Alarms Infra-red photography

Radiant heater

Uses of EM WavesVisible Light• Visible light is part of the electromagnetic spectrum that the human

eye can detect.• The various wavelength of light are classified by colours.

How to remember the colours of the visible light?

ROY G. BIV

Uses of EM WavesUses:

Optical Fibres for medical uses and telecommunications

Lasers for medical uses and telecommunications

Uses of EM WavesUltra-violet Radiation• Wavelengths range from 10–8 m to 10–7 m• Main source of ultra-violet radiation is sunlight

Uses:

Sunbeds

Sterilisation

Forgery detection

Fluorescence Effect

Uses of EM WavesX-Rays• wavelengths ranging from about 10-13 m (10-4 nm) to 10-8 m (10 nm)

Uses:

Medical/DentalInspection

Checking welds (for cracks)

Airport Security(Baggage check)

Radiation Therapy (Treatment of cancer)

Uses of EM WavesGamma Rays• Result of the decay of radioactive nuclei• Wavelengths range from about 10-10 m to less than 10-14 m. • Can cause serious damage when absorbed by living tissue• They can cause mutations which lead to cancer• Highest frequency highest energy

Uses:

Checking welds (for cracks)

Sterilising equipment

Radiation Therapy (Treatment of cancer)

Uses of EM WavesGamma Knife

The high energy gamma rays are emitted from a cobalt-60 source.

The rays are focused on the brain tumour through the use of a special protective helmet with holes drilled in it.

1

2

Each individual beam is not strong enough to damage normal living tissue.

3

At the point where the beams converge, the cumulative energy is able to kill the cancerous cells.

4

Effects of EM Waves

Effects of EM WavesIonising radiation on living matter Ionising radiation is radiation that has the energy to remove

electrons from atoms or molecules.

Exposure to ionising radiation can damage biological molecules and lead to abnormal cell division.

This may cause cancers and/or deformities to a developing foetus.

Radiation warning symbol

Effects of EM WavesInfrared Heating The emission of infrared radiation is what makes us feel warm when

we stand near a barbecue pit.

We feel warm because we absorb the infrared radiation.

Heat radiating from the charcoal is actually infrared radiation.

Practice Question 1The velocity of radio waves is 3.0 × 108 m s–1. A radio station is broadcasting at a frequency of 1.0 × 106 Hz. What is the wavelength of the radio wave?

𝑣= 𝑓 l3.0×108=1.0×106× l

l=3 .0×108

1.0×106

¿300m

Practice Question 2Which of the following options lists the members of the electromagnetic spectrum in order of increasing wavelength?

A Microwaves, ultraviolet, infrared, X-rays

B X-rays, ultraviolet, infrared, microwaves

C Ultraviolet, infrared, microwaves, X-rays

D Infrared, ultraviolet, microwaves, X-rays

Answer:

a) Write down the order:

b) Recall Gamma Ray has highest energy highest f lowest l

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Highest energyHighest fLowest l

Practice Question 3Name three regions of the electromagnetic spectrum with lower frequency than visible light.

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Highest energyHighest fLowest l

Practice Question 4Do gamma rays or radio waves have a higher speed when travelling in a vacuum?

Answer:

All electromagnetic waves have the same speed (3 × 108 m s−1) when travelling in a vacuum.

QUESTIONS?Question