Transcript T3 L8 TIR and optical fibres mk1

IGCSE Physics
Waves
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Lesson 8 – TIR and optical fibres
Aims:
•To recall the meaning of critical angle c
•To recall and use the relationship between critical
angle and refractive index: sin c = 1/n
•To describe the role of total internal reflection in
transmitting information along optical fibres and in
prisms.
•To understand the difference between analogue and
digital signals.
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When light passing out of the glass
block its angle increases
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No more than ninety degrees
As soon as the
angle of refraction
reaches 90, the
light can no longer
be refracted.
What happens to
the light? Well the
light is reflected
back inside the
material.
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Total internal reflection
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TIR
All the light
is now
reflected
back.
This effect is called Total Internal Reflection and can
occur in many different materials.
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This angle is called the critical angle ( c).
The angle at
which total
internal
reflection first
occurs is called
thecritical
i<c
angle.
Refraction
i=c
i>c
Total internal
reflection (TIR)
Different materials have different critical angles.
Diamond has the lowest at 24°, which is why it reflects
so much light.
Critical case
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Start at zero degrees and go up
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Critical calculations
The critical angle for a material depends upon the
refractive index. The higher the refractive index, the
lower the critical angle. It can be calculated using the
following formula:
Sin c = 1/n
Where:
n = Refractive index,
c = Critical angle at which TIR first occurs.
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Critical angle – Example 1
Calculate the critical angle for a glass block of
refractive index 1.45
sin (c) = 1 / n
sin (c) = 1 / 1.45
sin (c) = 0.69
c = 44º
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Critical angle – Example 2
Calculate the refractive index of a material where
TIR occurs at a critical angle of 37º
sin (c) = 1 / n
n = 1 / sin (c)
n = 1 / sin (37)
n = 1 / 0.60
n = 1.66
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Critical angle – Example 3
Material
Refractive index
Critical
angle
Glass
1.5
42°
Water
1.33
49°
Diamond
2.4
24°
The greater the refractive index, the smaller the
critical angle.
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Using TIR
A right angled prism
will bend light
through 90.
Two of these prisms
can be used to
produce a periscope.
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Back the way they came
A right angled prism
will also bend light
through 180.
This idea is used in
reflective clothes and
signs.
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Digital and analogue
Before we look at a very important
use of TIR we need to review digital
and analogue signals.
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Signals
• Transmitted signals can be either analogue or digital.
• Analogue signals normally take the form of a continuous
waveform like a sine wave. In general analogue signals
suffer from interference called ‘noise’ especially if the
signal is repeated during transmission. Speech and pictures
are analogue signals.
• Digital signals take the form of binary code either on or
off. Digital signals do not suffer from noise even when the
signal is regenerated and amplified during transmissions.
Morse code is a simple example of binary transmission.
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What is the difference between a digital signal and an
analogue signal?
Digital signals can only
be in one of two states:
0 or 1.
Analogue signals are a
continuously changing
variable.
What do they look like graphically?
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Digital signals
•To create our digital signal we use varying voltages to
represent one and zero.
•A positive voltage can be used for zero.
•A negative voltage can be used for one.
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Analogue signal
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Digital vs. analogue
Advantages of digital
Disadvantages
 Signals are clearer and less
susceptible to noise.
 Digital hardware is
expensive at the moment.
 Can be used quickly by
computers.
 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.
 Carry digital signals using
electromagnetic waves, which
travel at the speed of light.
 Carry much more information.
 Digital hardware is much
smaller.
 Easier to send over long
distances.
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Fibre optics
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Optical fibres
Light is refracted as it
enters the fibre.
Every time it tries to
leave it is reflected
back inside.
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TIR in liquids and tubes
The light is always reflected back into the material and
does not escape. Total internal reflection is used to send
signals along fibre optic cables for the Internet and TV.23
What are the applications of total internal reflection (TIR)?
Optical fibres, used in
communication, use TIR.
You could be asked to
draw on the path of the
beam in an exam.
1. A beam of light enters
the optical fibre.
2. It is refracted as it
enters the fibre.
3. It travels down the fibre through repeated TIRs.
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Endoscope
An endoscope uses total internal reflection to enable
a doctor to look deep inside the body. It enables key
hole surgery to take place.
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A fibre optic is a very
thin piece of glass. It is
so thin that once light
enters at one end, it can
never strike the inside
of the glass at an angle
less than the critical
angle. The light
undergoes total internal
reflection as it passes
along the fibre.
Fibre optic cable
Fibre optics have several advantages over normal electrical wires:
(1) They can carry much more information than a wire, (2) They do
not suffer from static and so give a clearer connection, (3) They
have no electrical resistance, (4) They pose no danger of an electric
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shock if they break.
Optical v Electrical
Optical
Electrical
Information
Carry more
Carry less
Attenuation
Less
More
Interference
No
Yes
Cost
More
Less
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Advantages of Fibre optics
•Low attenuation (signal loss) of wave means they
can go further.
•Small diameter of fibre for a high capacity
channel.
•Low cost of materials.
•Cables may be non-conducting so no shocks.
•High Security, it is hard to listen in on an optical
fibre.
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Disadvantages of Fibre optics
• Need for additional conducting members in cable
when electrical supplies are required for remote
terminals.
• They can be damaged by some ionising
radiations.
• Electrical cables are already in place.
• Fibres not directly suited to multiple-access use.
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Fibre optic pictures
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Summary – TIR and optical fibres
•When light passes out of a material it can be internally
reflected. Refractive index and critical angle are connected
by a formula: sin (c) = 1/n
•Optical fibres are thin pieces of glass or plastic that light
can travel through whilst being totally internally reflected.
•Analogue signals have constantly changing amplitudes, a
sine wave or sound wave are examples of analogue signals.
•Digital signals are either on or off – they have only two
values. Digital signals are less effected by noise when
regenerated they analogue signals. Digital signals can carry
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more information than analogue signals.