12. Infrared and Visible Waves v2.1.ppt

© Boardworks Ltd 2006
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Which is the odd one out?

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What are infrared waves?
We experience infrared waves everyday in the
form of heat.
infrared
microwaves
10mm 0.001 mm
0.1 mm 0.00001 mm
visible
light
wavelength of electromagnetic waves
Infrared waves have wavelengths between 1 mm (about the
size of a pin head) and about 0.0001 mm (about the size of
a cell).
Infrared radiation is a form of electromagnetic
radiation that lies between microwaves and
visible light on the electromagnetic spectrum.

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What produces infrared waves?
Infrared radiation is emitted by all
objects but is not visible to humans.
The hotter an object is, the more
infrared radiation it emits.
Infrared radiation can be detected
by special cameras. A thermogram
shows how objects with different
temperatures emit different
amounts of infrared radiation.
Here, the white areas are warmest
and the blue areas are coldest.
As an object gets hotter, visible light
is also emitted. This is what can be
seen when an object glows ‘red hot’.

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How are infrared waves used?
Infrared waves have various uses. How many can you spot?

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How is infrared used for heating and cooking?
The infrared radiation emitted by the
Sun, radiators, solid fuel fires and
electric fires is used for heating.
Grills, cookers, toasters,
campfires and barbecues use
infrared radiation to cook food.
The heating effect of infrared
radiation is detected by temperature
sensitive nerve-endings in skin.
Infrared waves are emitted by
the heating element or lit fuel
and absorbed by the surface of
the food. Heat is transferred
through the food by conduction.

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How do infrared waves cook food?

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How is infrared used in remote controls?
Not all infrared radiation is thermal. Infrared radiation with
short wavelengths are not hot and cannot be detected by skin.
This type of infrared radiation
is used by remote controls to
send information through the
air over short distances.
Infrared waves can also be used to transmit information
through optical fibres. In this way, infrared radiation can send
data over long distances and around corners.
The pulses of infrared
radiation from the remote
control can only travel in
a straight line to the
device being controlled.

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An infrared thermometer enables the remote sensing of
temperature. It does this by detecting the infrared radiation
emitted by an object and converting it into a temperature.
How do infrared thermometers work?
The eardrum is an accurate point for
measuring the body’s temperature,
because it is deep within the head.
A digital ear thermometer can
do this without touching the
eardrum, which is very fragile.
A probe is inserted into ear canal
and then measures the infrared
radiation emitted from the
eardrum and surrounding tissues.

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Devices containing infrared detectors are
used for various security purposes.
How can infrared waves fight crime?
Sensors in security lights and burglar alarms
can detect the infrared radiation given off by
an approaching body.
Thermal imaging equipment, as
used by the military and police,
detects infrared radiation and
converts it into an image. Objects
such as people and cars, that are
hotter than the background can
be easily tracked even at night.
The emergency services use thermal imaging cameras to
help locate people in situations where visibility is low.

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Uses of infrared – activity

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What is light?
Every time someone opens their eyes, they are
making use of ‘visible’ light.
visible
light
infrared
0.1mm 0.00001 mm
0.001 mm 1x10-7 mm
ultra-
violet
wavelength of electromagnetic waves
Light waves have wavelengths between 390 and 780 nm.
The different wavelengths of light are seen as the different
colours of the spectrum.
Light is the only type of electromagnetic radiation
that is visible to eye. It lies between infrared and
ultraviolet radiation on the electromagnetic spectrum.

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What is light used for?
 Human sight
Human eyes are sensitive to visible light.
The different wavelengths of light are
detected as different colours.
 Imaging
Photography uses light-sensitive
chemicals to produce images. Digital
cameras contain sensors that detect light
and use this to produce electrical signals.
 Navigation
Navigation lights are used at sea to mark
hazards and safe areas. Lights are also
displayed by boats and aircraft, at night, to
show their position and direction of travel.

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How is light produced?
Visible light is emitted by very hot objects.
Neon lights contain gases that
emit visible light when an
electrical current is passed
through them. These emit light
of certain wavelengths, that is
seen as different colours.
The filament of this light bulb reaches a
temperature of 2500°C and glows ‘white
hot’, making it a source of light.
The Sun, our nearest star, is a natural
source of light. This light is part of the
energy produced by the nuclear fusion
reactions that take place in the Sun.

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How else can light be produced?
Chemical reactions can also emit
energy as visible light. This is
called chemiluminescence.
Some biological organisms have
the ability to produce light. This
is bioluminescence.
Certain chemicals can emit visible
light when struck by ultraviolet
radiation. This process is called
fluorescence.
Fluorescent tubes contains mercury vapour, which generates
ultraviolet radiation an electrical current is passed through it.
This UV radiation hits the fluorescent coating on the inside of
the tube and creates visible light .

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Visible light – source or use?

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What is total internal reflection?
When a light ray hits the boundary
between two materials of different
densities (e.g. glass and air), the
ray is normally bent or refracted.
If the angle of incidence of the light
ray is greater than a specific value,
called the critical angle, then the
light ray is actually reflected. This is
called total internal reflection.
This occurs because the speed of
light changes in different materials.
It makes the inner surface of
glass act like a perfect mirror.

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Total internal reflection – simulation

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Why is angle of incidence important?
If the angle of
incidence is
smaller than
the critical angle,
then the light ray
is refracted.
If the angle of
incidence equals
the critical angle,
then the light ray
is refracted along
the boundary.
If the angle of
incidence is greater
than the critical
angle, then the light
ray is not refracted
and total internal
reflection occurs.

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Is the critical angle always the same?
The critical angle is the smallest angle of incidence at which
total internal reflection occurs.
Diamond has the lowest critical angle at 24°.
49°
42°
41°
24°
water
acrylic plastic
glass
diamond
Critical angle
Material
Different materials have a specific value for the critical angle:
This means that diamond reflects more light than the other
materials and accounts for its characteristic sparkle.

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Total internal reflection activity

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Is communicating with light a new idea?
Light has been used to send messages throughout history.
For example, the Egyptians used sunlight to communicate.
A visual signalling device called
an Aldis lamp was widely used
by the British Navy to send
messages over short distances.
Today, computers encode information as digital signals,
which can be sent along optical fibres using light.
The messages were translated
into Morse code. The encoded
messages were transmitted
using a flashing signal lamp.
Short flashes to represent ‘dots’
and long flashes for ‘dashes’.

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Optical fibres are thin strands of
solid glass, about the size of a
human hair.
What are optical fibres?
They are widely used in
communication, medicine,
lighting and as sensors.
The first transatlantic telephone
cable to use optical fibres went
into operation in 1988.
Optical fibres can transmit light
signals at high speed over long
distances and are used in
phone and internet connections.

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Why are optical fibres so important?

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Optical fibres do not have to be straight to carry light and can
even carry light around corners.
How do optical fibres carry light?
Light travels through the Perspex rod, and optical fibres, by a
process called total internal reflection.
This curved Perspex
rod shows how light
travels in an optical
fibre.

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What is total internal reflection?
A ray of light enters the optical fibre.
light enters
optical fibre
light leaves
optical fibre
As the light enters the optical
fibre, it is refracted. This means
that the direction the light is
travelling in changes.
The ray of light hits the wall
of the fibre and is totally
internally reflected when the
angle of incidence is greater
than the critical angle.
The ray of light passes down
the optical fibre by repeated
total internal reflection.

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What’s in a cable of optical fibres?

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Optical fibres can be used by doctors to look inside a
patient’s body without having to cut them open to check that
they are healthy.
The device used for this
purpose is called an
endoscope. It contains a
bundle of optical fibres that
are held together and can
be inserted into the body.
How are optical fibres used in medicine?
Light is sent down some of
the fibres and the image is
reflected back through the
other fibres.

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Optical fibres – true or false?

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Glossary
 critical angle – If light hits a boundary between two
materials at an angle greater than this value, then total
internal reflection occurs.
 infrared radiation – Electromagnetic radiation that lies
between microwaves and visible light. It is involved in heating.
 optical fibres – Long, thin strands of glass or plastic used
to carry digital signals as pulses of infrared or visible light.
 total internal reflection – An optical effect that occurs
when light hits a boundary between two materials at an angle
greater than the critical angle, so that all the light is reflected.
 visible light – The only type of electromagnetic waves
that can be detected by the human eye.

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Anagrams

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Multiple-choice quiz

12. Infrared and Visible Waves v2.1.ppt

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