P1 – Energy for the
home
Revision lesson 2
The Electromagnetic
Spectrum
Each type of radiation shown in the electromagnetic spectrum has a
different wavelength and a different frequency:
Gamma rays
X-rays
Ultra violet
Visible light
Infra red
Microwaves
Radio/TV
γ
High frequency,
_____ wavelength
All waves travel at 300 000 000m/s in a vacuum
Low frequency, _____
(high) wavelength
Sending signals by light, electricity and radio and
microwaves all have various advantages and disadvantages.
Light signals?
Can be easily sent with nothing more sophisticated
than a torch, but the signal needs to be coded and is
not secure.
Electrical signals? (eg house telephone)
Need equipment and wires linking the sender and receiver
of the signal. Wires can be cut or damaged. Signal needs
to be amplified at regular intervals because of loss of
energy in form of heat due to the electric current.
Radio waves and microwaves?
(eg satellite, mobile, radio)
These can travel very large distances and through the
atmosphere – and even across space!
Comparing infra-red and microwave
cookery
Higher
Infra-red cooked food is usually browned, Microwave cooked food is
This is because of the way the energy is transferred.
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Infra-red:
Energy is absorbed by the particles on the
surface of the food.
This increases the kinetic energy of the
particles. The energy is then transferred to
the rest of the food by conduction.
Microwaves:
the water molecules in the outer layers
vibrate more and their kinetic energy
increases.
Which cooks quickest?
The energy is then transferred to the rest of
the food by convection and conduction
Electromagnetic Spectrum
- To do
1. Which parts of the spectrum can be used in
communication?
2. What are the concerns about using mobile
phones?
3. Which two waves in the spectrum can be used to
cook food. Describe the differences between
the two methods
1. Radio, microwaves and light
2. If microwaves are absorbed by water molecules
in living tissue, cells may be burned/killed.
Potential dangers are prolonged exposure but
there is no conclusive evidence
3. Microwaves and Infrared . Microwaves heat
water molecules in food and IR heats the
surface of the food
Reflection, Refraction and
Diffraction
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all electromagnetic waves can be reflected, refracted and
diffracted.
Reflection:
The wave changes direction at a boundary. It returns in the
direction it originated.
Refraction:
A change in direction due to the speed of the wave. (caused
by entering a different medium; e.g. light from air into water)
Diffraction:
Bending of waves caused by obstacles. The degree and type
of diffraction depends on the obstacle
Reflection
Angle of incidence = Angle of reflection
Normal
Reflected ray
Incident ray
Angle of incidence
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Angle of reflection
Mirror
Refraction through a glass block:
Wave slows down and bends
towards the normal due to
entering a more dense medium
Wave slows down but is
not bent, due to entering
along the normal
Wave speeds up and bends
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away from
the normal due to
entering a less dense medium
Wave Properties
1) Reflection
2) Refraction
3) Refraction
4) Diffraction
Diffraction – Higher only
More diffraction if the size of the gap is similar to the wavelength
More diffraction if wavelength is increased (or frequency decreased)
Diffraction depends on frequency…
A high frequency (short
wavelength)microwaves -wave doesn’t get
diffracted much – the house won’t be able
to receive it…
Diffraction depends on frequency…
A low frequency (long wavelength) radio
waves wave will get diffracted more, so
the house can receive it…
Sending information - Finding the
Critical Angle…
1) Ray gets refracted
3) Ray still gets refracted (just!)
THE CRITICAL
ANGLE
2) Ray still gets refracted
4) Ray gets
internally reflected
Uses of Total Internal Reflection
Optical fibres: Use total internal reflection to send
information
Uses – communication systems, endoscope, binoculars
Light Amplification by Stimulated Emission of Radiation
White light from a bulb is made up of many different colours.
Each is of a different frequency and is out of PHASE.
The light from a LASER is at just one frequency and is in
phase. This makes it very much brighter and able to travel
further that ordinary white light.
Lasers produce narrow, intense beams of monochromatic light
Higher - Compact Discs
• Plastic disc with a billions of pits on the bottom shiny
surface (coated with metal film)
• The pits represent a digital signal
• The signal is read by a laser
• The shiny metal film reflects
the laser light
Questions
1.
2.
3.
1.
Name 2 uses of lasers.
Why is it dangerous to look at a laser beam?
What is morse code and why is it important?
communication, dental treatment, surgery,
light shows, bar code readers
2. Narrow intense beam of light capable of
cutting, burning or vaporising materials
3. Digital signal - Series of dots and dashes
representing letters of the alphabet. Used to
send messages
6 mark question
• The quality of written communication will be
assessed in your answer to this question.
• Describe how light signals can travel
through optical fibres and why they are
important
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Level 3
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Answer correctly applies knowledge of how light signals bouncing waves
off the sides of narrow core, they enter the fibre so it hits the boundary
between the core and the outer cladding at an angle greater than the
critical angle, causing total internal reflection. Pulse of light enters at one
end and is reflected again and again until it emerges at the other end
.Uses – telephone conversations and computer data All information in
answer is relevant, clear, organised and presented in a structured and
coherent format. Specialist terms are used appropriately. Few, if any,
errors in grammar, punctuation and spelling. (5–6 marks)
Level 2
Answer applies knowledge of how light signals can pass through an
optical fibre and why they are important. For the most part the information
is relevant and presented in a structured and coherent format. Specialist
terms are used for the most part appropriately. There are occasional errors
in grammar, punctuation and spelling. (3–4 marks)
Level 1
An incomplete answer, states that light signals are totally internally
reflected and mentions a use. Answer may be simplistic. There may be
limited use of specialist terms. Errors of grammar, punctuation and spelling
prevent communication of the science. (1–2 marks)
Level 0
Insufficient or irrelevant science. Answer not worthy of credit. (0 marks)
Analogue vs. Digital
Analogue signals (like talking or
music) continually vary in
amplitude and/or frequency
1
0
+
Digital signals, however, are either
off or on, and the information is sent
in a series of pulses
There are two main advantages of digital:
1) More information can be sent down the same cable
2) Better quality, because a digital signal can be amplified without
amplifying the extra noise:
Transmitting information – Higher
As we said before, different types of electromagnetic radiation can be
used to send different types of information, e.g. an optical fibre:
Optical fibres have two main advantages: they can send more information
compared to electrical cables of the same diameter and with less signal
weakening.
Another example is radio waves:
Ionosphere
The short wave radio (TV and FM) and medium radio waves are transmitted
by being reflected off the ionosphere (an electrically charged layer in the
Earth’s atmosphere).
Long wave radio waves diffract (bend) around the Earths surface
Uses of radio waves
• terrestrial
frequency
Ultra
waves
Very
curvature
distant
ionosphere
transmit
long
high
Longer
Earth
• Radio waves are used to __________ radio and television
programmes between different points on the
__________’s surface. __________ wavelength radio
waves are reflected from an electrically charged layer in
the Earth’s upper atmosphere, which is called the
__________. This enables them to be sent between
__________ points despite the __________ of the
Earth’s surface. __________ __________ frequency
(VHF) radio waves allow higher quality audio transmission
than __________ waves or medium __________
__________ high __________ (UHF) radio waves are used
for television and by the police.
Multiplexing:
higher
This allows many different signals to be
transmitted simultaneously, with less interference
Each digital signal is divided into segments of very
short duration.
A combined signal takes each segment in turn and transmits
it. A multiplexer combines the individual signals to be sent
and a demultiplexer separates them at the receiving end of
the transmission.
demultiplexer
Signal A
multiplexer
Signal A
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
Signal B
bbbbbbbbbbbbbbb
Signal C
cccccccccccccccc
Transmitted as
abcabcabcabcabcabcabcabc
Signal B
bbbbbbbbbbbbbbb
Signal C
cccccccccccccccc
Used in telecommunications and computer networks
Multiplexing and the digital switchover
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Due to the large amount of data that can be
sent due to multiplexing, the switch from
analogue to digital has been made possible.
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The main benefits of digital television and radio
are:
Improved signal quality (sound and image)
Improved choice of programmes
Interaction with programmes (e. g. red button
Programme guides
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6 mark question - Higher
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The quality of written communication will be
assessed in your answer to this question.
 Lisa
lives in a house in the valley. He
can listen to long-wave and medium
wave stations on his radio. He can’ t
get any signal on his mobile phone
and VHF (very high frequency used
for FM) reception is very poor.
Explain these differences
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Level 3
Answer correctly applies knowledge of how long and medium wave
signals diffracted around the obstacle to his house; some diffraction with
VHF; however microwaves have an even shorter wavelength so do not
diffract as much. Microwave transmitter needs to be in the line of sight. All
information in answer is relevant, clear, organised and presented in a
structured and coherent format. Specialist terms are used appropriately.
Few, if any, errors in grammar, punctuation and spelling. (5–6 marks)
Level 2
Answer applies knowledge of how microwaves do not diffract as much as
radio waves therefore it can’t be received at the house behind the hill, and
how the transmitter needs to be in the line of sight. For the most part the
information is relevant and presented in a structured and coherent format.
Specialist terms are used for the most part appropriately. There are
occasional errors in grammar, punctuation and spelling. (3–4 marks)
Level 1
An incomplete answer, states that radio waves diffract more than
microwaves . Answer may be simplistic. There may be limited use of
specialist terms. Errors of grammar, punctuation and spelling prevent
communication of the science. (1–2 marks)
Level 0
Insufficient or irrelevant science. Answer not worthy of credit. (0 marks)
Questions
What type of EM wave is used in
satellite communication?
2.
Give 2 disadvantages of the use of
microwaves in mobile phones
3. What are digital and analogue signals.
What is the advantage of sending
information digitally?
4.
Give 3 uses of IR.
5. Why is it said that texting is a safer
way to communicate than making a call
from a mobile phone?
1.
Answers
1.
2.
3.
4.
5.
Microwaves
Don’t diffract as much due to short wavelength so
transmitters need to be in line of sight, interference
between signals (affected by weather)
Digital – on/off, analogue can take any value. Advantage –
better quality, send several signals at the same time
(multiplexing)
Security systems, remote controls, mobile phones and
computers (only short distances though and must point
directly)
There has been reports that mobile phones may be
dangerous if absorbed by water molecules in living tissue,
cells may be burned. However there is no conclusive
evidence, if there was surely mobile phones would be
designed to give out less microwaves and there would be a
legal age limit for use or use for emergency calls only
Term
Meaning
A radio wave
1 distance occupied by one complete cycle of a wave
B refraction
2 the shortest wavelength radio waves in the electromagnetic
spectrum
C frequency
3 the complete range of observed frequencies of
electromagnetic waves
D microwaves
4 the change in direction of a wave when it passes from one
medium to another of different density
E aerial
5 poor radio reception caused by overlapping waves
F ionosphere
G interference
6 part of the electromagnetic spectrum used mainly for
communication
7 a layer of the upper atmosphere that reflects some radio
waves
H wavelength
8 the number of complete waves passing a point in 1 second
I electromagnetic
spectrum
9 effect of two signals from the same source which have
travelled different distances to reach a television aerial
J ghosting
10 device for receiving or transmitting radio signals
Some definitions…
Transverse waves are
when the displacement
is at right angles to the
direction of the wave…
Longitudinal waves
are when the
displacement is
parallel to the
direction of the wave…
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)
The Wave Equation
The wave equation relates the speed of the wave to its
frequency and wavelength:
Wave speed (v) = frequency (f) x wavelength ()
in m/s
in Hz
in m
V
f

Some example wave equation questions
1) A water wave travels through a pond with a speed of
1m/s and a frequency of 5Hz. How far apart are the
waves?
2) The speed of sound is 330m/s (in air). When Ricky
hears this sound his ear vibrates 660 times a second.
What was the wavelength of the sound?
3) Purple light has a wavelength of around 6x10-7m. If its
frequency is 5x1014 Hz what is the speed of light?
4) Red light travels at the same speed. Work out its
frequency if its wavelength is about 4x10-7m.
The Structure of the Earth
A thin crust 10-100km thick
A mantle – has the
properties of a solid
but it can also flow
A core – made of
molten nickel and iron.
Outer part is liquid
and inner part is solid
How do we know this? These facts have all been
discovered by examining seismic waves (earthquakes)
Seismic waves
Earthquakes travel as waves through the Earth – we call them
SEISMIC WAVES. There are two types:
P waves:
1) They are longitudinal so they cause the ground
to move up and down
2) They can pass through solids and liquids
3) They go faster through more dense material
S waves:
1) They are transverse so they cause the ground
to move from right to left
2) They ONLY pass through solids
3) They are slower than P waves
4) They go faster through more dense material
Waves through the Earth
- Questions
1. How do we know the structure of the Earth?
2. Describe two differences between p and s waves
3. Name one advantage and disadvantage of
seismometer.
1. How Seismic waves, such as P and S waves travel through
the Earth
2. S waves can only travel through solids and are slower than
p waves. P waves can travel through solids and liquids. S
waves are transverse and P waves are longitudinal
3. Advantage e.g. seismometers detect p waves before
more damaging s waves arrive so people can get to a safe
place. Disadvantage – they record information after an
earthquake has happened so may not give much warning
Human effect on the
Earth
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Use of CFC’s in aerosols has meant that the
ozone layer has become damaged. This
means that more of the Sun’s harmful U.V.
is reaching us
Burning fossil fuels is producing carbon
dioxide (a greenhouse gas). This is
trapping in the heat and contributing to
global warming
Avoiding Sunburn
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Sun index gives information about the strength
of the sun and how long you can stay in the sun
without burning if you are not wearing sunscreen.
Sun protection factor (SPF) is an indication of
how much longer you can stay in the Sun when
using sunscreen; SPF 10 means 10 times longer,
etc.
SPF  safe time given by sun index = time you can
stay in the Sun with that SPF sunscreen.
Quick question
What is the maximum amount of
time you could spend in the sun if
the sun index is 8 and you put on sun
cream with a SPF factor of 8?
Safe time = SPF no. X time given in
sun index
8 x
20-30 min
= 160 min – 240 min
Questions
What can happen when a person is exposed to
too much UV radiation?
2.
Why are people with darker skin less likely to
suffer from skin cancer?
3.
What does a SPF 15 mean?
4.
Why are scientists concerned about the thinning
of the ozone layer?
5.
UV radiation can damage the DNA in your cells,
which may lead to skin cancer, it can cause eye
problems such as cataracts, as well as premature
skin aging
2. The skin absorbs more of the UV so less reaches
the cells that may become cancerous
3. You can stay out in the sun 15 times as long
4. Ozone layer has got thinner due to the overuse of
CFCs, this allows more UV rays to reach us at the
surface of the Earth
1.
Plenary - To Do
Produce a summary sheet / mind map
for the P1 – Energy for the Home
topic