1. KS4:
Electromagnetic
Waves

2. Electromagnetic Spectrum
 When particles [usually electrons] accelerate or
decelerate, they make electromagnetic waves.
 Electromagnetic waves are transverse waves made up
of electric and magnetic fields which travel together.
 All electromagnetic
waves can travel through
space.
 All electromagnetic waves
travel at the same speed
[300,000,000 m/s in a
vacuum].

3. Electromagnetic Spectrum
 Visible light is
made when negative
electrons slow down
as they move around
inside an atom.

4. Electromagnetic Spectrum
 Although all e-m waves travel at the same speed,
their wavelength [λ] and frequency [ƒ] can change.
 The properties, dangers and uses of e-m waves
depends on the wavelength [λ].
Waves that cook
food.
Waves that cause
sun-tans.
Waves that cause
cancer.

5. Electromagnetic Spectrum
 The whole family of electromagnetic waves is called
the electromagnetic spectrum.
Radio
Micro
Infra-Red
Visible
Ultra-Violet
Xrays
Gamma
λ increases
ƒ increases

6. Electromagnetic Spectrum
Name Gamma Rays
λ 0.000 000 001 mm
Properties
 Very high energy
 Pass through
body unchanged
 VERY dangerous
Uses
 Kill cancer cells
[radiotherapy]
 Sterilise medical
equipment
 As tracers to look
at lung structure

7. Electromagnetic Spectrum
Name X rays
λ 0.000 001 mm
Properties
 Energetic
 Short wave X rays
pass through
flesh but not bone
 Dangerous
Uses
 Look through
body i.e. broken
bones and teeth
 Scan luggage for
dangerous items

8. Electromagnetic Spectrum

9. Electromagnetic Spectrum
Name Ultra Violet
λ 0.000 01 mm
Properties
 λ is too short for
eyes to see
 causes sun-tans
Uses
 ‘sun’ beds
 checking
counterfeit
banknotes
 in nightclubs
The £5 note on the
left is genuine.
The note on the
right glows in UV
and is counterfeit

10. Electromagnetic Spectrum
Name Visible Light
λ 0.005 mm
Properties
 Our eyes respond
to these λ
 Made of ROYGBIV
Uses
 To see!
 Expose
photographic film
 To generate
electricity in photo-
electric cells [solar
panels]

11. Electromagnetic Spectrum
Name Infra Red
λ 0.01 mm
Properties
 Emitted by warm
objects
 Hotter the object,
shorter λ emitted
Uses
 Remote controls
 Thermal imaging
cameras
 Electric grill

12. Electromagnetic Spectrum
Name Microwaves
λ 1 mm – 1 cm
Properties
 Can carry
information
 Some λ absorbed
by food
Uses
 Microwave ovens
 Mobile
communications
 RADAR
Microwaves reflect
around the oven and
carry energy to
about 1 cm into the
food. This cooks
the food quickly

13. Electromagnetic Spectrum
Name Radio
λ 10 cm – 1 km
Properties
 Can carry
information
 Travel large
distances
Uses
 Broadcasting
[carry radio and
TV signals]
96.7 FM
Radio waves are
emitted when
electrons move up
and down an aerial
very quickly.

14. Electromagnetic Spectrum
1) Match up the following parts of the electromagnetic
spectrum with their uses :
Gamma rays Allow us to see
Radio waves Remote Controls
Ultra Violet ‘See’ broken bones
Visible Carry TV signals
Microwaves RADAR
X rays Sterilise equipment
Infra Red Causes sun-tans

15. Electromagnetic Spectrum
1) A radio station uses waves of frequency 96.7 MHz
If the speed of e-m waves in air is 300,000,000 m/s,
a) calculate the wavelength of the radio waves
used.
b) calculate the time taken for the transmission
to travel 50 km.
2) Why can we see the Sun but can’t hear it?
3) Write down 3 things all e-m waves have in common.

16. Reflection : A reminder
 From KS3 you should remember :
 Pale and shiny surfaces are good reflectors,
dark and rough surfaces are not.
 The image in a plane mirror is laterally inverted.
 The image is the same distance behind the
mirror as the object is in front.
 The image in a plane mirror is the same size as
the object.
 angle of incidence = angle of reflection
¡ = r

17. Reflection : A reminder
Angle i
Angle r
¡ = r
Incident ray
reflected ray

18. Reflection : Curved Mirrors
 In KS3 you just dealt with plane mirrors.
 By curving a mirror, we can make mirrors more
useful:
Concave
mirrors
curve
inwards
Convex
mirrors
bulge
outwards

19. Reflection : Curved Mirrors
ƒ
Chose a
distant object
[to get
parallel rays
of light].
Finding ƒ of a concave mirror.
Hold the
mirror in the
other hand and
move it closer
to the screen
until a clear
image appears.
Hold a plain
white
screen in
one hand.
Use a ruler to
measure the
distance between
the lens and the
screen - this is
the focal length
[ƒ].

20. Reflection : How does curvature
affect ƒ ?
 Concave mirrors reflect rays of light to a focal
point.
 The distance between the mirror and the focal
point is called the focal length [ƒ].
 How can ƒ be changed?
 Concave mirrors produce real images because the
rays of light meet [unless the object is close].

21. Reflection : How does curvature
affect ƒ ?
ƒ
 Take a piece of
Al or stainless
steel sheet and
curve it slightly.
 Shine parallel
rays of light at the
reflector and plot
their positions.
 Draw around
the reflector.
 Measure ƒ and record your results.
 Carefully bend the reflector and repeat the process to
see how ƒ changes with curvature.

22. Reflection : Convex mirrors
ƒ
 Convex
mirrors
reflect
rays of
light away
from a
focal
point.
 The
distance
between
the
mirror
and the
focal
point is
called
the
focal
length
[ƒ] Convex mirrors produce virtual images - the
rays of light do not meet.

23. Reflection : Curved mirrors
 Concave reflectors
are used to focus signals
from distant satellites.
 Convex reflectors
are used to widen the
field of view.

24. Total Internal Reflection
Incident
ray
Reflected
ray
Refracted
ray
Angle i Angle r
Refraction or
Reflection?
15°
30°
45°
60°
75°
Angle i
Angle r
Angle r
 At what angle of incidence did the ray change from
refraction to reflection?

25. Total Internal Reflection
 This angle is called the critical angle [c]
i < c
Refraction
i = c
Critical case
i > c
Total Internal
Reflection
[TIR]
 Different materials have different
critical angles - diamond has the lowest at
24º which is why it reflects so much light.

26. Total Internal Reflection
i = r
Optical fibre

27. Total Internal Reflection
 Why do communications systems now use
optical fibres instead of copper wires?
ADVANTAGES
 Can carry much more
information as digital
signals.
 Carry information at
the speed of light [300,
000 km/s].
 Clear signals
unaffected by electrical
interference.
DISADVANTAGES
 Expensive to make
as very high quality
glass is needed.
 Need careful
handling - signal loss if
cracked.

28. Refraction : A reminder
 When light bends this is called refraction.
 Refraction happens because the light changes
speed [or velocity].
 If the incident ray hits a surface at 0º, no
refraction occurs.
air
glass

29. Refraction : Lenses
 At KS4, you need to be able to explain how to
change the size and nature of an image formed
by a convex lens.

30. Refraction : Lenses
1. Find the focal length [ƒ] of your lens.
2. Fix the lens to the centre of a metre rule and
mark the distances F and 2F either side of the lens.
2F F F 2F
3. Place the candle >2F away from the lens and move
the screen until an image appears.
4. Measure the distances between the candle, image
and lens and describe the image in the results table.

31. Refraction : Lenses
Object
position
[as F]
Distance
from O to
lens [cm]
Image
position
[as F]
Distance
from I to
lens [cm]
Image
Descrip
tion
Graph
>2F
away
2F away
between
F & 2F
at F
between
F and
lens
Magnif
ication

32. Refraction : Lenses
 Object >2F away
O
2F F F 2F
I
 The image [ l ] is formed between F and 2F away
from the lens, is inverted and diminished.

33. Refraction : Lenses
 Object at 2F
O
2F F F 2F
I
 The image [ l ] is formed at 2F away from the lens,
is inverted and the same size.

34. Refraction : Lenses
 Object between 2Fand F away
O
2F F F 2F
I
 The image [ l ] is formed further than 2F
away from the lens, is inverted and magnified.

35. Refraction : Lenses
 Object at F away
O
2F F F 2F
 The image [ l ] is formed at infinity - the rays
never meet [we use this set-up for searchlights].

36. Refraction : Lenses
 Object between F and lens
O
I
 The VIRTUAL image
[ l ] is formed on the
same side of the lens as
the object, is the right
way up and magnified.
2F F F 2F

37. Refraction : Lenses
Magnification = Distance from lens to image
Distance from object to lens
2F F F 2F

38. Using Refraction : Sound
 Sound waves can be refracted as well as light waves
 Move the
microphone across
the balloon and
watch the CRO
trace of the sound
wave.
 What does the
CO2 in the balloon
do to the sound
waves? Why?
CO2

39. Diffraction & Interference
 Waves travel in straight lines but when they go
past an edge they spread out in a new direction.
 This is called diffraction

40. Diffraction & Interference
 When 2 waves meet, they interfere with each other.
 If they meet each other exactly in phase, the
amplitudes ‘add up’ to produce large crests and troughs.
+ =
 This is called constructive interference.

41. Diffraction & Interference
 If they meet each other exactly out of phase, the
amplitudes ‘subtract’ to produce no peaks or crests.
+ =
 This is called destructive interference.

42. Diffraction & Interference
 To get 2 waves of light to interfere, the waves must
be very similar.
 We use a single source of
monochromatic light and split
it into 2 waves using a diffraction
grating like this:
 In 1801, a physicist called Young first
performed this classic investigation which
showed the interference of light waves.

43. Diffraction & Interference
The light
source
emits rays
of light
which
diffract
towards
the double
slit
S1
S2
S1 and S2 act as 2 coherent light sources
 The waves interfere - constructively [bright fringes].
destructively [dark fringes].
Fringes

44. Diffraction & Interference
 What would the fringes look like if white light
was used as the source instead?

45. Diffraction & Interference
 The coloured
fringes on these
CDs are the result
of interference.
Light reflecting
from the
Aluminium
diffracts and
interferes.
Some colours are
diffracted more
than others.

46. Communication
Communicate v. Make known; transmit; pass
information to and fro; have means of access
 To pass information quickly over large distances, we
use waves.
These dishes collect and
focus microwaves from a
communications satellite
100’s of km above the
Earth.
 The effects of reflection, refraction and
diffraction are important to consider when designing
communications systems.

47. Communication
The meters
at the top are
analogue
meters - they
use a needle
to represent
the reading.
The digital
meters
below give
the reading
as a number.
 Computers handle digital readings much faster and easier.
3.4 2.6

48. Communication
0
1
2
3
4
5
6
0 1 2 3 4 5
Time [1/10,000 s]
SamplingLevel
 Use the chart on the next page to turn the analogue
signal into binary code and then a voltage sequence.

49. Communication
Sampling
Level
Binary
Code
Voltage Sequence
0 000 LOW LOW LOW
1 001 LOW LOW HIGH
2 010 LOW HIGH LOW
3 011 LOW HIGH HIGH
4 100 HIGH LOW LOW
5 101 HIGH LOW HIGH
6 110 HIGH HIGH LOW

50. Communication
ADVANTAGES
 Signals are clearer.
 Can be used quickly by
computers.
 Carry digital signals
using electromagnetic
waves which travel at the
speed of light.
 Carry much more
information.
 Digital hardware is
much smaller.
DISADVANTAGES
 Digital hardware is
expensive at the moment.
 Although digital signals
are unaffected by
electrical interference,
they don’t give a complete
signal [just lots of
samples] - some people
feel that analogue vinyl
records sound better than
digital CDs for this reason.

51. Communication
ionosphere
Transmitter
dish
Receiver
dish
Gugliemo Marconi first reflected radio waves off
the ionosphere in 1901 [from England to Canada].

52. Communication
Transmitter
dish
Receiver
dish
The UHF radio waves we use for TV carry a lot of
information but don’t reflect off the ionosphere.
We use communications satellites which amplify and
transmit the signal.

53. Communication : Diffraction
 UHF radio waves carry high quality TV signals but
can’t diffract round hills very well - you get a poor signal
in valleys.
 LW and MW signals diffract round hills so you get a
good signal in valleys.

54. Communication : Diffraction
 Waves from the transmitter dish spread out due to
diffraction.
 The receiver dish can’t collect all the waves and so
some energy is wasted - the signal must be amplified.