The sources and properties of infrared
radiation (thermal radiation)
To obtain a viable source of FM infrared radiation ('heat
radiation') all you need is an energy store at a higher temperature than the
background e.g. hot water radiator, electric heater
The Sun is the most powerful emitter of
infrared radiation (thermal radiation) - energy is being continuously
released by nuclear fusion of hydrogen to helium, so it isn't cooling down!
When you heat up
materials the bonds between the atoms in the molecules vibrate more
energetically, and so the molecules are more 'energetic' with respect to the cooler
less 'vibrating' background molecules.
When the vibrations decrease as the
particles 'relax' to their normal energy levels, the energy
is released by the material emitting FM infrared radiation.
(There is also heat transfer by
conduction to any cooler material in contact with the hotter material.)
Infrared radiation is absorbed directly by molecules
- increasing their kinetic energy of movement/vibration and so increasing
the energy store of the absorbing material.
All materials are continually emitting and absorbing
infrared radiation and the hotter the material the more infrared radiation
it emits.
Uses of infrared radiation
An electric heater energy
store transfers and emits infrared radiation to warm you up and increase your thermal energy
store.
The
hot surface of radiators emit infrared radiation (but there is also
conduction from the hot water or electrical heating element to the surface,
so heat is also conveyed away by convection currents in the air).
All 'hot
heaters' transmit infrared EM radiation to increase the thermal energy store
of the surroundings increasing its temperature.
When you grill food e.g. toasting bread,
you are using infrared radiation to raise the temperature of the food - the
surface of the food absorbs the radiant energy from the toaster's heating
elements, from one thermal energy store to another.
The electrical resistance elements of a cooker
ring or a toaster become hot enough to emit a strong beam of infrared
radiation to heat the contents of a pan (left) or grill the toast
(right).
Electrical energy is converted into
thermal energy which increases the
thermal energy store of the heating elements, some of which is converted
to infrared
radiation, which on absorption, increases the thermal
energy store of the pan and contents or bread being toasted etc.
Note on cooking techniques
(i) When food is grilled,
initially only the surface is cooked, because infrared is not very
penetrating and deeper inside the food will be less cooked - perhaps
not sufficiently for health and safety.
(ii) In microwave cooking, the
radiation can penetrate deep into the food and quite quickly too
(often just a few minutes), so the food is more thoroughly cooked,
but not necessarily as tasty, since we like 'fried' food.
(iii) When food is cooked in an
oven, plenty of time is usually allowed for the heat to conduct
right through the food e.g. baking bread at 180oC for
minutes.
Remote controls
for TV, DVD players, garage door and curtain control in a house!
Infrared signal devices are used
as remote controllers for many household appliances and in industry too.
The
instructions are encoded in the infrared beam. Such devices work by sending out
a different signal pattern for each particular command eg for a TV and
recorder, each channel, stop, pause, play etc. will have their own unique
code transmitted in the infrared signal.
In a similar manner, infrared
beams can be used to transfer files between mobile phones or
laptops. However, the distance between the devices must be short and
the receiver must be in the direct line of sight of the transmitter.
You can design infrared security
systems
(i) You can set up an infrared
emitter and detector system that triggers an alarm if the signal is
interrupted (blocked) by an unwanted intruder on a property.
(ii) Thermal imaging security cameras
work well at night, when normal visible light cameras give poor imaging
e.g. if there was a night-time break-in at your home, thermal imaging
could provide accurate important video evidence of the intruders for
both the police and your home insurance provider.
Infrared can be used to transfer
information e.g. multiple telephone calls or TV signals through optical fibres
at nearly the speed of
light!
Optic fibres are thin glass or
plastic strands that you can send a signal through carrying information
e.g. from computers, telephones and other data transfer systems.
The IR waves just bounces off the side of the thin strands of the
glass fibres (known as 'total internal reflection') and travels unimpeded
down the optical fibre with little loss due to absorption or scattering
of the wave energy on the side of the fibre optic cable.
The IR signal is transmitted into the optical
fibres, travels to the ends of them, and the signal picked up by a receiver.
Optical fibres can transfer information over very long distances.
Optical fibres often use a single
visible light wavelength/frequency carrier wave to reduce loss of information.
The digital information
signal is imposed on this infrared radiation carrier wave.
Cable television is delivered in this way.
Infrared cameras detect IR radiation and
build up a 'temperature picture' of what's in focus a bit like a visible
light camera does. The technique is called
thermal imaging.
The infrared radiation is converted into an electrical signal and
displayed on a screen.
You still see the shapes of objects but they are all
contoured in different colours depending on the temperature of the surface.
The hotter the object's surface is the brighter it appears on the
screen - you get contours of bands of different surface temperatures.
You photograph a house with an
infrared camera and detect where most heat loss is occurring.
Firefighters can use thermal
imaging cameras to look for the infrared emitted by warm bodies of
unconscious people in smoke-filled buildings - visible light is absorbed
or scattered by smoke particles, infrared is more penetrating.
Unlike visible light cameras, IR cameras
work off 'invisible' infrared radiation, and can be used in night-vision
cameras - security, nocturnal wildlife photography.
An increasingly important use is for
infrared cameras on low level orbiting satellites to monitor the use of
land e.g. crops, deforestation and the growth of urban areas -
particularly fast growing cities.
Different surface radiate or
absorb different amounts of infrared producing contours of slightly
different temperatures.
Heat sensors can detect infrared radiation -
safety device warning of overheating.
A greenhouse traps infrared radiation.
The higher frequency (shorter wavelength) infrared from the Sun passes
through the glass of the greenhouse warming the contents. The contents
re-radiate infrared radiation of lower frequency (longer wavelength) that
does not pass through the glass as easily, so more of the heat is trapped in
the greenhouse.
Narrow high intensity beams of infrared
can be used to cut through sheets of metal.
Infrared radiation can be used in medical imaging, rather like the night vision camera, they detect areas
on the body that have increased in temperature due to an infection - again,
your are dealing with different intensities of infrared radiation producing
contours of slightly different temperatures. The technique also has the
advantage of monitoring the temperature over a wide region of the body very
quickly - no need for a thermometer.
Unfortunately, despite being safe to
use, infrared radiation is of very limited use in diagnostic medical
imaging.
Another 'domestic' case of infrared radiation! Unlike 'modern'
LED bulbs, 'old fashioned' filament bulbs emit quite a bit of IR heat
radiation. You can detect this with a frosty car where the central portion
of the ice has melted on the transparent headlamp cover. Filament bulbs only
convert ~10% of the electrical energy into visible light energy, most of the
rest is converted into infrared radiation. The ice absorbs infrared
equivalent to the latent heat of fusion (melting) and changes to liquid
water.
Dangers of infrared radiation
Most infrared radiation is reflected or
absorbed by the skin. Infrared radiation is readily absorbed by
your skin and at high intensity will cause burns - from over exposure to
sunlight or too close to a radiant fire.
Infrared radiation contributes to
'heatstroke'/'sunstroke' when the body temperature rises over 40oC
(104oF).
This can occur in hot ambient
conditions, particularly if you are in bright sunshine and dehydrated.
Your body temperature is usually close to 37oC (98.6oF),
but in extreme conditions your
thermoregulation system
can fail. Initially you feel unwell (because you are!) and other
symptoms are confusion, red skin, headache and dizziness.
For humans and other warm-blooded
animals, excessive body temperature can disrupt enzymes regulating
biochemical reactions that are essential for cellular respiration and
the functioning of major organs
