Infrared Radiation - Electromagnetic Waves

The visible light that we see every day is really a small portion of the electromagnetic spectrum. The electromagnetic spectrum includes all types of radiation ranging from the X-rays used at the hospitals, to radio waves used for communication.
Electromagnetic Spectrum
Radiation in the electromagnetic spectrum is categorised by wavelength. Short wavelength radiation such as Gamma, X-rays and ultraviolet is of high energy and can be very dangerous. Longer wavelength radiation such as radio, microwaves and infrared are less harmful.  In this article, we will be discussing infrared radiation and its characteristics in detail. Although infrared radiation is not visible, humans can sense it as heat. To experience infrared radiation “first-hand, put your hand next to a hot oven!

What is Infrared Radiation?

Infrared radiation (IR), sometimes known as infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. Hence, it is undetectable by the human eye, although IR of wavelengths up to 1050 nanometers (nm)s from specially pulsed lasers can be seen by humans under certain conditions. Infrared light extends from the suggested red edge of the visible spectrum at 700 nanometers to 1 millimetre. Most of the thermal radiation emitted by objects near room temperature is infrared. As with all EMR, IR carries radiant energy and behaves both like a wave and like its quantum particle, the photon. Depending on the wavelength and frequency, infrared is commonly divided into five categories as near-wavelength, short-wavelength, mid-wavelength, long-wavelength and far-infrared.

Characteristics of Infrared Radiation

Infrared Radiation
Infrared Radiation from the Sun

  • Infrared radiations are also referred to as heat or thermal waves or electromagnetic waves. This is because they have a heat-inducing property. Sometimes infrared rays are used in applications where heat production is required like in infrared heaters or for therapeutic purposes where a patient requires physical therapy.
  • Sometimes infrared rays themselves are classified as near-infrared and far-infrared rays.
  • Near-infrared rays are used in electronic applications like TV remote sensors and photography. Their applications can be somewhat similar to visible light applications since their wavelength ranges are close.
  • Far infrared rays are more thermal in nature. Anything generating heat gives out far-infrared radiation. Even the human body (at \(37^{\circ}C\)) gives off infrared radiations of around 800 nm wavelength.

How was Infrared Radiation Discovered?

William Herschel
William Herschel (1738-1822)
According to NASA, infrared light was discovered by British astronomer William Herschel in 1800.  In an experiment to measure the temperature difference between the colours in the visible spectrum, he placed thermometers in the path of light within each colour of the visible spectrum.  He noticed an increase in temperature from blue to red, and he found an even warmer temperature measurement just beyond the red end of the visible spectrum. Within the electromagnetic spectrum, infrared waves occur at frequencies above those of microwaves and just below those of red visible light, hence the name “infrared.”

The wavelength of Infrared Radiation

We already know that the wavelength of the infrared radiation is between 700 nm to 1 mm, which is between the red limit of the visible spectrum. But following is the classification of bands based on the spectral range 1µm and 50µm:

  • 1µm to 3µm which is known as the Band I or Short Wave Infrared
  • 3µm to 5µm which is known as the Band II or Middle Wave Infrared
  • 8µm to 14µm which is known as the Band III or Long Wave Infrared

Characteristics of Regions of Infrared

Near-infrared 
  • The wavelength ranges from 0.75 to 1.4 micrometres.
  • This is used in material science, fibre optic communication, and in the medical field.
Short wavelength infrared
  • The wavelength ranges from 1.4 to 3 micrometres.
  • This is used in telecommunications and for military purposes.
Medium infrared
  • The wavelength ranges from 3 to 8 micrometres.
  • This is used in the chemical industry and in astronomy.
Long-wavelength infrared
  • The wavelength ranges from 8 to 15 micrometres.
  • Astronomical telescopes and optical fibre communications are possible with the help of long-wavelength infrared
Far infrared
  • The wavelength ranges from 15 to 1000 micrometres.
  • It is used mainly in the treatment of cancer therapy.

Properties of Infrared Waves

Much of the energy from the sun reaches the Earth in the form of infrared radiation. The balance between absorbed and emitted infrared radiation has a critical effect on the Earth’s climate. Below, we have listed the properties of Infrared waves.

Transverse Waves

According to Serway’s College Physics, an infrared wave is said to be a transverse wave, i.e., the displacement of the wave is in right angles to the direction of the wave propagation.

Wavelength

The wavelengths of infrared waves are unique and are usually measured in microns. A micron is defined as one-millionth of a metre. The shortest wavelength of an infrared wave is about 0.7 microns. The longest wavelength of an infrared wave is 350 microns. According to studies, the upper limit of any infrared wave is 1000 microns.

Speed

The speed at which infrared waves travels is 299,792,458 m.s-1.

Particle or Wave?

According to quantum theory, infrared waves can exist as either wave or as a particle at the same time.

Absorption and Reflection

The absorption and reflection of infrared waves depend on the nature of the substance that the waves are made to strike. Materials such as ozone, carbon dioxide, and water vapour absorb infrared radiation. Snow and aluminium foil are the materials that reflect the infrared radiation.

Refraction and Interference

Infrared waves exhibit the property of refraction making the waves experience a slight change in direction when the wave passes from one medium to another. Refraction property of infrared wave can be noticed in the earth’s atmosphere. When two infrared waves with the same wavelength meet each other, they will interfere with one another.

Thermal Properties

Infrared radiation can be the source of heat as they have thermal properties. When the infrared radiation strikes the oxygen or nitrogen molecules, the radiation makes the molecules move faster as they gain more energy. So it can be concluded that infrared radiation makes materials hotter and therefore can be used as a heat source.

Where do we use Infrared Rays?

We make use of infrared rays in the following:

  •  Communications: Infrared lasers are used for free-space optical communications as these are relatively inexpensive. These also find applications in the areas that are densely populated.
  • Spectroscopy: Infrared vibrational spectroscopy is used for identifying the molecules by analyzing their bonds as the molecule vibrates at different frequencies. The unit used to express the radiation is cm-1.
  • Astronomy: Optical components like mirrors, lenses and digital detectors are used to study the objects that are in space. These objects glow when they are exposed to the radiated heat.

Applications of Infrared Waves

Following are the areas of use of infrared waves:

Heat Source

There are two different industries that use infrared radiation as a heat source and they are:

  • Medical uses: Infrared radiation finds application as a heating source. There are infrared saunas which are used for treating high blood pressure, rheumatoid arthritis, and high blood pressure which are chronic health problems. Infrared radiation is considered to be one of the safest methods of physiotherapy.
  • Manufacturing industries: Industries which concentrate on manufacturing processes such as forming of plastics, curing of coatings, plastic welding, etc use infrared heaters in place of contact heating and convection ovens.

Cosmetology Application

Infrared rays are widely used for cosmetic applications such as for treating skin injuries, smoothing of wrinkles, reducing the occurrence of dandruff, blackheads, etc. The reason why infrared rays are used is that they can penetrate through the skin up to 3-4 mm. They also warm the skin resulting in improved blood circulation, continuous supply of oxygen and other nutrients to the skin.

Astronomy

Astronomers use optical devices such as mirrors, solid-state digital detectors, lenses to study objects from space with the help of infrared waves. The images from these optical devices are obtained with the help of an infrared telescope.

Massage Therapy

Infrared rays are used for warming the skin and for relaxing the muscles. Infrared rays are preferred because of their penetration quality through the skin.

Infrared Photography

Infrared filters are used for capturing pictures in infrared photography. This imaging is done for objects that are placed in the near-infrared spectrum. Most the digital cameras use infrared blockers making the near-infrared appear as purple-white colour in the final image.

An infrared photograph of a tree

Infrared Communication

Data transmission with the help of infrared radiation is very common in short-range communication. For encoding the data infrared light-emitting diodes are used which emit infrared radiation and is focused into a narrow beam with the help of a plastic lens. At the receiver end, a photodiode is placed for converting infrared radiation into electric current.

Interesting Facts About Infrared Radiation

Who discovered infrared radiation?

Sir Frederick William Herschel discovered infrared radiation.

When was infrared discovered?

Infrared was discovered in the year 1800.

What does infrared mean?

The term infrared is a Latin word in which infra means below. Also, red is the colour with the longest wavelength in the visible spectrum. Therefore, it is known as infrared.

Name the infrared radiation measuring instrument.

Infrared radiation is measured using an instrument known as an infrared thermometer.

How does infrared radiation work?

The emission of infrared radiation from an object is possible when it is heated such that the atoms and the molecules in the object start to vibrate thereby radiating infrared in the form of heat. When the objects are not hot enough to produce visible light they radiate infrared. Also, heat production is independent of the temperature of the surroundings. 

Extended Reading List

Frequently Asked Questions on Infrared Radiation

List a few properties of infrared radiation.

Following are a few properties of infrared radiation:

  1. Infrared radiation, like all radiation, travels at a speed of 299,792,458 meters per second.
  2. Infrared light can exhibit both wave and particle nature at the same time.
  3. Depending on the nature of the material that infrared radiation strikes, it can either be absorbed or reflected.
  4. Infrared radiation increases the thermal energy in particles.

List a few medicinal effects of infrared radiation.

A few medicinal effects of infrared radiation are as follows:

  • Used to reduce the inflammation of throat, ear and nose
  • Used as an aid in stress management
  • Reduces pain arthritis
  • It eases kidney function

How do we experience infrared radiation on a daily basis?

Infrared is a type of radiant energy that is invisible to the eyes and we can only feel it in the form of heat. All objects in the universe emit some level of IR radiation. Sun and fire are among the most obvious source of infrared radiation.

How to select infrared detectors?

How to select infrared detectors?

Following are the guidelines that need to be followed when infrared detectors are selected:

  • The temperature and wavelength of the object need to be measured.
  • Photosensitivity and signal to noise need to be noted. Also, for the signal noise reduction, the infrared detector must be cooled using either thermoelectric cooling or mechanical cooling or by cryogenic cooling.
  • Depending on the application of the infrared detector, response speed and chopping frequency need to be recorded.
  • When infrared detectors are used in the optical systems, the size and the geometry of the active area plays a vital role.
  • Infrared detectors are available in various packages and these packages are metal can package and ceramic packages.

What is the classification of the infrared detector?

The infrared detector is classified into two:

  • Thermal type: Heat supply to thermal type detector happens through the infrared energy and the photosensitivity is independent of the wavelength. The thermocouple is an example of a thermal type infrared detector.
  • Quantum type: This can be either an intrinsic type or extrinsic type. The detection performance is higher when compared to the thermal type. Intrinsic type is further divided into two and they are photoconductive type and photovoltaic type. Photosensitivity of these detectors are dependent on the wavelength.

What are the characteristics of infrared radiation?

  • Invisible to human eyes: This is considered to be one of the characteristics of infrared radiation and is also useful for security purposes. The drawback of invisibility of infrared radiation to the human eye is that designing of optical instruments becomes difficult.
  • Small energy: The energy of infrared radiation is equal to the vibrational and rotational energy of the molecules. This characteristic is used in the identification of the molecules.
  • Long-wavelength: The transmission becomes better as there is less scattering through the medium.
  • Infrared radiation can be emitted from all kinds of objects.

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