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Electromagnetic spectrum: 5. The properties, uses and dangers of infrared waves (IR thermal radiation)

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INDEX of physics notes: Properties and uses of electromagnetic radiation


5. The properties, uses and dangers of infrared waves (IR radiation)

See also Introduction to heat transfer - including infrared radiation and

Absorption & emission of radiation by materials - temperature & surface factors including infrared

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

 

See also Introduction to heat transfer - including infrared radiation and

Absorption & emission of radiation by materials - temperature & surface factors including infrared

INDEX of notes: Properties and uses of electromagnetic radiation


Keywords, phrases and learning objectives for electromagnetic radiation spectrum

Be able to describe the properties of infrared (thermal) radiation and sources of infrared forming part of the electromagnetic spectrum.

Be able to describe the uses of infrared radiation including thermal imaging cameras, fibre optics for carrying communication signals, cooking food grill, radiator heaters, video/TV remote control,  medical imaging from temperature differences on the surface of the body,

Know the dangers of intense infrared radiation causing skin burns.


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INDEX of notes: Properties and uses of electromagnetic radiation

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