#### Transcript 10. Electromagnetic waves

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What are electromagnetic waves?
What’s the connection between
light, microwaves and X-rays?
They are all different types of
travel as waves and transfer
energy from one place to another.
magnetic
field
electric
field
Electromagnetic waves are
of electric and magnetic fields.
All electromagnetic waves
travel at the same speed.
wave
direction
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In a vacuum (space), they
travel at 300,000,000 m/s!
Some definitions…
1) Amplitude – this is “how
high” the wave is:
2) Wavelength () – this is the distance
between two corresponding points on the
wave and is measured in metres:
3) Frequency – this is how many waves pass by every second and is
measured in Hertz (Hz)
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How do electromagnetic waves differ?
Different electromagnetic waves
carry different amounts of energy.
For example, microwaves carry
less energy that X-rays.
 The amount of energy carried by an electromagnetic wave
depends on the wavelength:
the shorter the wavelength, the higher its energy.
 Wavelength and frequency are linked properties of a wave:
the shorter the wavelength, the higher its frequency.
 So, frequency also tells you about the energy of a wave:
the higher its frequency, the higher the energy.
Do microwaves have a shorter wavelength than X-rays?
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What happens when waves hit a surface?
When electromagnetic waves hit
a surface, they can be reflected,
absorbed or transmitted.
How the waves behave, depends on
their energy and the type of material.
For example, light waves are
reflected by skin but X-rays pass
straight through.
If electromagnetic waves are
absorbed, some of their energy is
absorbed by the material. This
usually increases the temperature
of the material.
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What happens when waves are reflected?
Some surfaces can reflect
electromagnetic waves. Shiny
surfaces are good reflectors
of light waves.
When waves are reflected,
some of their energy may also
be absorbed by the material.
A mirror reflects most of
the light waves that hit it.
The curved satellite dish
reflects microwaves from
How does reflection allow
us to see?
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What is the electromagnetic spectrum?
The electromagnetic waves are grouped into types that
have similar wavelengths and so have similar properties.
waves microwaves
100 m
10 cm
infrared
visible
light
1 mm
0.01 mm
ultraviolet
100 nm
X-rays
1 nm
gamma
rays
0.01 nm
smaller wavelength
higher frequency, energy and hazard.
Electromagnetic waves form a continuous series in order
of changing wavelength, frequency and energy.
This series is called the electromagnetic spectrum.
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The electromagnetic spectrum
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Wavelength of electromagnetic waves
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Are electromagnetic waves dangerous?
The shorter the wavelength (and higher the frequency) of
electromagnetic waves, the more energy that they carry.
waves microwaves
100 m
10 cm
infrared
visible
light
ultraviolet
1 mm
0.01 mm
100 nm
X-rays
1 nm
gamma
rays
0.01 nm
energy increases
hazard increases
High-frequency electromagnetic waves, such as gamma rays,
are potentially more harmful because they have more energy.
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Energy of electromagnetic waves
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How do radio waves affect humans?
Radio waves are the longestwavelength electromagnetic waves
and mostly pass through the body.
They are not strongly absorbed
and are thought to have no effect
on the health of living tissue.
short wavelengths. They are very
slightly absorbed by the body and
can cause a minor heating effect.
However, the microwaves
produced by mobile phones
have not yet been proved to
cause health problems.
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How do infrared waves affect humans?
Infrared waves are absorbed
by skin to a limited depth. They
transfer their energy to the skin
tissue warming it up.
This heating effect is detected
by temperature-sensitive nerve
endings in the skin.
Infrared waves from this grill
heat the surface of the meat.
If the meat absorbs too much
energy, it will become burnt.
If skin is exposed to too many
high-energy infrared waves,
it will become burnt.
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How does visible light affect humans?
Your eyes detect visible light, which does
not normally pose any health risk.
However, very bright light can damage
your eyes and may even make you blind.
This is why you should not look at the
Sun through a telescope or binoculars.
Lasers are very intense sources of
visible light. The lasers used in light
shows are not powerful enough to
cause harm but must be used safely.
Some very powerful lasers can cut
through materials such as metal.
These would also be able to burn
through living tissue.
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How do ionizing waves affect humans?
Ionizing waves have enough energy to ionize the atoms in
materials. These waves can have a severe effect on living
tissue by killing cells or damaging DNA.
 Ultraviolet rays are absorbed by the
body. Skin tissues can be ionized and
damaged by the shortest-wavelength
 X-rays pass through soft body tissue,
such as skin and muscle, without being
absorbed. Denser tissue, such as bone,
absorb some X-rays and can be ionized.
 Gamma rays pass through the body
but very high energy waves can ionize
atoms in living tissue.
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Effect of electromagnetic waves
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Glossary (1/2)
 absorption – The process in which electromagnetic waves
hit a surface and their energy is transferred to the material.
 electromagnetic spectrum – The continuous series of
electromagnetic waves in order of wavelength or frequency.
 frequency – The number of waves passing a point every
second, measured in hertz (Hz).
 gamma rays – Electromagnetic waves with the shortest
wavelengths. Uses include sterilizing medical equipment and
cancer treatment.
 infrared waves – Electromagnetic waves given off by hot
objects. Uses include heating, cooking and remote controls.
 microwaves – Electromagnetic waves that are shortwavelength radio waves. Uses include mobile phones, radar
and cooking.
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Glossary (2/2)
 radio waves – Electromagnetic waves with the longest
wavelengths. Used for communications.
 reflection – The process in which electromagnetic waves
bounce back from a surface.
 wavelength – The distance between two matching points
on neighbouring waves, which is measured in metres.
 ultraviolet rays – Electromagnetic waves that can be
absorbed by certain chemicals and emitted as visible light.
Uses include tanning beds and security marking.
 visible light – Electromagnetic waves that are detected
by our eyes, allowing us to see. Other uses include optical
fibre communications and imaging.
 X-rays – Electromagnetic waves that pass through most
materials. Uses include medical imaging and security.
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Anagrams
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Multiple-choice quiz
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The electromagnetic spectrum
How many different electromagnetic waves can you spot?
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