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Physics > Infrared

The term 'Infrared radiation' covers a part of the electromagnetic spectrum that we cannot see. It comprises wavelengths that are longer than visible light but shorter than radio waves. Infrared literally translated means 'below red'. Fig 1 below shows a diagram of the electromagnetic spectrum showing where infrared radiation appears.

Fig 1: Simplified electromagnetic spectrum
Fig 1:
Simplified electromagnetic spectrum

When infrared is discussed, many people instantly think of night vision. Filming animals at night on the African plains or firefighters seeing through smoke both use infrared radiation but in different ways.

To understand how, you must first know that infrared radiation can be split into roughly three categories depending on its wavelength. Other subdivisions are sometimes applied but may not be precise and vary from author to author, but to make things simple:

  • Near Infrared (near-IR) has wavelengths ranging from 0.75 to 5 micrometers and is the nearest to visible light.
  • Mid infrared (mid-IR) has wavelengths ranging from 5 to 30 micrometers. It is this frequency range that is commonly used in remote controls and other electronic applications.
  • Thermal infrared (thermal-IR) frequencies range from 30 to 1000 micrometers.
Objects that consume energy such as humans or non-living objects like rockets and engines create heat. This heating involves atoms getting excited and emitting photons in the thermal infrared frequency range. A very hot object will begin to emit photons in the visible spectrum turning from red, through yellow to white.

Thermal imaging uses thermal IR detectors to build up a detailed picture of the object being viewed from the thermal IR radiation it is emitting. This is how firefighters 'see' through dense smoke using thermal detecting cameras.

CCTV type cameras which see in the dark use 'near infrared' lights, usually in LED form, to illuminate the objects being filmed. This IR radiation is reflected back from the object into the camera where it is picked up by the image sensor. Using IR illumination in this way is like using a torch in the dark.

Many cheaper digital cameras and camera phones can actually 'see' infra red radiation that is emitted from remote controls etc. If you press a button on any IR remote control and aim the lens of the camera at the emitter LED, you should see a blue to white beam appear on the digital viewfinder. This is pretty impressive to see and shows pupils that although IR may be invisible to the naked eye, it behaves just like visible light. More expensive cameras tend to use IR filters which don't allow any IR to get through to the photo sensor.

The thermal infrared frequencies are often referred to as being 'heat' radiation and it is these frequencies that IR 'heat lamps' emit. Many schools have these lamps for heating purposes, showing how white and black metal sheets for example absorb its radiant heat. For more information see Equipment > Infrared Heat Lamp or Practicals > Simple Thermal Radiation

Infrared thermometers are used in some schools to detect the infrared radiation being emitted from objects. Particularly useful in showing how different coloured metal surfaces radiate more heat than others. These thermometers are more accurate in 'reading' temperatures from smooth surfaces although highly reflective or shiny metallic surfaces may give slightly skewed results although they are a lot more accurate than if a standard mercury thermometer is used against a surface. For more information see Equipment > Thermometers.