The Electromagnetic Spectrum by Year 12 Physics Students

Infrared radiation

By Abinayan Rasaratnam 12G and Alex Pregal 12I

This is a report on what infrared waves are, their discovery, the physics behind them, the benefits, uses, dangers and risks they pose and also their uses.

What is infrared?

Infrared radiation is invisible radiant energy in the form of transverse waves and is a part of the electromagnetic spectrum. The waves are in the region between microwaves and the red part of visible light, which means they have wavelengths of between 700nm-1mm and frequencies between 430THz-300GHz. They are broken into three categories: near, mid and far-Infrared.

Near-Infrared refers to the part of the Infrared spectrum that is closest to visible light. Near IR waves are not actually warm, but they do exist. These are the ones used by your TV remote.

Far-Infrared refers to the part that is closer to the microwave region. Far-Infrared waves are thermal and these are the waves we experience every day in terms of heat e.g. when you turn on a lamp and the bulb heats the surroundings.

Mid-Infrared is the region between these two.

Infrared radiation is most commonly known as heat radiation, as most of the thermal radiation emitted by objects at room temperature is infrared, however all of the electromagnetic waves can emit thermal radiation, while those with very short wavelengths, such as X-rays, will emit a much greater amount of heat. Thermal radiation is only popularly associated with infrared as most heat on earth is very comparatively very low and as such caused by infrared.

The waves can also travel through a vacuum as they do not need a medium to travel through unlike sound waves. As with all em waves they have a speed of 3 x E8 m/s in a vacuum.

Commonly used sub-division scheme

Near-infrared 0.75–1.4 µm 214-400 THz 886-1653 meV Defined by the water absorption, and commonly used in fibre optic telecommunication because of low attenuation losses in the SiO2 glass (silica) medium. Image intensifiers are sensitive to this area of the spectrum. Examples include night vision devices such as night vision goggles. It is also used in remote controls, astronomy, remote monitoring, material science, medical field and agriculture.

Short-wavelength infrared 1.4-3 µm 100-214 THz 413-886 meV Water absorption increases significantly at 1,450 nm. The 1,530 to 1,560 nm range is the dominant spectral region for long-distance telecommunications. It is also used in SWIR cameras, night vision goggles that play a major role for military purposes

Mid-wavelength infrared – Also called intermediate infrared (IIR) 3–8 µm 37-100 THz 155–413 meV In guided missile technology the 3–5 µm portion of this band is the atmospheric window in which the homing heads of passive IR ‘heat seeking’ missiles are designed to work, homing on to the Infrared signature of the target aircraft, typically the jet engine exhaust plume. This region is known as thermal infrared, but it detects only temperatures somewhat above body temperature. Also used for infrared spectroscopy, communication, and the chemical industry. It also finds application in astronomy.

Long-wavelength infrared 8–15 µm 20-37 THz 83–155 meV The “thermal imaging” region, in which sensors can obtain a completely passive image of objects only slightly higher in temperature than room temperature – for example, the human body – based on thermal emissions only and requiring no illumination such as the sun, moon, or infrared illuminator. This region is also called the “thermal infrared.” This finds extensive application in astronomical telescopes and optical fibre communication.

Far-infrared 15–1,000 µm 0.3-20 THz 1.2–83 meV Used in infrared lasers, astronomy, infrared saunas and extensively used in the medical field. It strengthens the immune system.




Infrared waves were discovered by William Herschel, a German-born British astronomer, in the early 19th century. Herschel published his results in 1800 before the Royal Society of London. Herschel used a prism to refract light from the sun, to disperse the white light into the colours of the spectrum. He then observed a temperature increase, using a thermometer, just beyond the red end of the spectrum. Surprised by his findings, he called them “Calorific Rays” (the term “infrared” didn’t appear until late in the 19th century).



Sir Frederick William Herschel, KH, FRS (15 November 1738 – 25 August 1822) was a German-born British astronomer, composer, and brother of Caroline Herschel. Born in the Electorate of Hanover, Herschel followed his father into the Military Band of Hanover, before migrating to Great Britain at the age of nineteen.

Physics behind infrared

Infrared waves are emitted by all warm objects. When they make contact with other objects, they cause them to heat up, as the waves of energy are absorbed by the molecules of the object which causes them to vibrate more and heat up. This in turn means the object will then emit infrared radiation. An example of this is during the day, the sun emits infrared radiation which heats up the earth as it absorbs the infrared. Then during the night, the earth re-emits the infrared radiation out into its atmosphere.


Infrared radiation is divided into regions which have different wavelengths and have different applications. The different regions are near-infrared (NIR), mid-infrared (MIR) and far-infrared (FIR). These regions have wavelengths of around 0.7-3 micrometres, 3-50 micrometres and 50-1000 micrometres respectively.

Properties of Infrared radiation

•Infrared radiation can travel in vacuum at the speed of light.

•Infrared radiation can travel through thick fog, thick smoke, dust and some other materials through which visible light cannot travel.

•Infrared radiation heats up objects on which they fall

•Infrared rays can be absorbed or reflected depending on the nature of the substance they fall on.

•Infrared has wavelength longer than visible light and shorter than microwaves

•Infrared radiation does not involve particles.

•Infrared radiation is emitted by all objects above zero degrees Kelvin (zero degrees kelvin is absolute zero which is -273.15 degrees Celsius or -459.67 degrees Fahrenheit and is the temperature below which all atomic and molecular motion stops). So even ice that we consider as a cold object emits infrared radiation.

•The higher the temperature of any object, the more infrared radiation it will emit.

•Infrared radiation cannot be seen but can be felt as heat.

•The infrared region close to visible light (near infrared region) is not warm and this is used in remote controls and for data transmission between a computer’s peripheral devices.

•Human body naturally produces infrared within the tissues which help with healing and cell repair

Detection of Infrared radiation

Infrared radiation can be detected by infrared detectors that react to infrared radiation. One can feel infrared radiation by its property of heating up the skin and other objects. Infrared imaging cameras are a good example for recording infrared images. There are some electronic devices that can detect infrared radiation and they have to be maintained at a lower temperature in order to avoid heat from the device interfering with the detection of infrared.

Some devices that are used to detect infrared radiation are:

•Thermocouple detectors, Bolometers – Heat sensitive devices that react to the presence of IR radiation

•Photovoltaic cells, photoconductors – made of semiconductor materials. The electrical conductivity of these materials increase when exposed to infrared radiation thereby helping with the detection of IR radiation

Uses of infrared

Infrared radiation is invisible but the energy of this region happens to relate to the approximate level of energy needed to start molecules moving in various ways. For example, wavelengths in the middle of the IR region are able to start various portions of molecules vibrating. This is the part of Infrared where it is used for cooking.

Infrared has many different applications, some of which are night vision, thermography, tracking, heating, communications, astronomy and meteorology. Infrared can be used in wired communications as it can be totally internally reflected just like visible light. This means it can be used alongside visible light to send data across optical fibres to maximise the amount of data that can be transmitted simultaneously. Far infrared (FIR) is the region which causes heat, and it can be used to cook food in toasters or ovens and also dry your hair with a hair dryer. Scientific study also shows that FIR can be beneficial to human health. FIR is said to be able to improve blood circulation, modulate sleep, ease pain, protect against oxidative stress and relieve inflammation.

We even use infrared every day, because we operate a TV or any device with a remote, and remotes have near infrared. DVD and Blu-ray players also have infrared in the form of a laser, which allows any disc to be read and transform into digital signals. We also use Infrared in cooking and it’s mostly found in ovens, this heats up all the food molecules, because it can transfer enough energy to break chemical bonds

You’ve probably seen TV programmes in which police helicopters track criminals at night, using thermal imaging cameras, which can see in the dark. These cameras use Infrared waves instead of ordinary light, which is why people look bright in these pictures. Similar cameras are also used by fire crews and other rescue workers, to find people trapped in rubble.


Weather forecasters use satellite pictures to see what’s heading our way. Some of the images they use are taken using IR cameras, because they show cloud and rain patterns more clearly.



One unusual form of Infrared radiation which can be immediately hazardous is a particular type of laser pointer. Most handheld laser pointers sold to consumers are essentially safe since, as someone accidentally looks into the laser, they will automatically shut their eyes in response to the bright light before the eye can be damaged.

However, tests have shown that some inexpensive laser pointers which emit green light also emit invisible Infrared laser light at a powerful level enough to damage the eye. Since Infrared light is not visible it does not cause a blink reflex and these lasers could potentially be harmful.

Risks of infrared

In certain industries, there can be very strong infrared radiation which can be very dangerous to human eyes, resulting in damage or blindness. Since infrared is invisible, special IR-proof goggles must be worn in places where very intense infrared radiation is present.

Even though infrared light is sometimes referred to as infrared radiation, it is not particularly harmful in most cases. This is because, unlike more powerful forms of radiation, Infrared light only has enough energy to start molecules moving, not to break them apart or otherwise cause damage in the way that higher energy waves such as X-rays do.

The one exception is that workers who are exposed to close, high levels of Infrared over many years, like those working with molten glass or steel, have been found to have increased incidences of eye cataracts as a result.

Long exposure of infrared can make someone’s body absorb Infrared light and the only consequence is usually that the person feels warmer.

1. Life Extension Magazine – June 2009 – Far Infrared Therapy Capturing the Therapeutic Benefits of Solar Energy – by Stephen Laifer

2. Edexcel AS Physics – Student’s Book – Page 105 Authors: Miles Hudson and Patrick Fullick ISBN: 978-1-4058-9638-2

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