The Electromagnetic Spectrum by year 12 physics students

Infrared Radiation

Amis Shanaz 12Y


Infrared was discovered in 1800 by astronomer William Herschel, the discovery of infrared depended on the use of detectors which did not include the human eye which is why Herschel described the new found radiation as ‘invisible light’ since it was not visible to human eyes.

Physics Review- Volume 4- Number 3- January 1995- Page 12 by Michael Barnett


Frederick William Herschel, KH, FRS (Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a Hanoverian-born British astronomer, technical expert, and composer.

Sir William conducted an experiment to measure the temperature difference between the colours in the visible spectrum. His results showed that the temperature increased from blue to red, he then noticed that readings beyond the red end of the visible spectrum had higher temperatures and this is how Infrared was discovered.

Details of Infrared

Infrared light is electromagnetic radiation with longer wavelengths than that of visible light , it extends from the nominal red edge of the visible spectrum at 750 nm to 1 mm, this means that the wavelength corresponds to a frequency of range of 300 GHz to 430 THz.


The above diagram shows the size scale of infrared, the temperature and also the wavelength in relation to the rest of the electromagnetic spectrum.

Edexel AS Physics By Miles Hudson and Patrick Fullick (ISBN:978-1-4058-9638-2) Page 105.

The primary source for infrared radiation is heat or thermal radiation; this is produced by the motion of atoms and molecules in an object. Any object with a temperature (i.e. anything above 0℃) radiates in infrared. At normal body temperature humans radiate mostly infrared at 10 microns. The diagram below shows the red areas as the warmest then yellow green and blue (being the coolest).


Common uses for Infrared

Infrared is used in a wide range of areas such as car locking systems, navigation systems, telephones etc. It is also used to discover new things about our universe. It is used in cameras by the police, the security services, the fire service and the military as infrared imaging detects heat loss/gain in buildings and people. It is used for testing electronic systems.

Infrared waves travel through glass in a similar way to visible light. Infrared can undergo total internal reflection (when the entire light incident at the boundary between a denser and a less dense medium is reflected back into the denser medium) to get carried along optical fibres.


n is the symbol for refractive index. For total internal reflection the refractive index of the first medium is greater than the refractive index of the second medium (n1>n2)

– The angle of incidence must be greater than the critical angle (i>c)


Benefits and risks

Infrared is around us all the time so ultimately it is not going to harm us however there is a chance you could burn your hand if you placed it next to something very hot. It could also be potentially dangerous if highly concentrated into a narrow beam (that is the principle of lasers) of very high power.

You can do a version of Herschel’s experiment to see how infrared was discovered. Use a slide projector (or any other source of a light beam) to shine light through a prism to make the spectrum. Connect a temperature sensor to a data logger, which is in turn connected to a computer. Move the sensor across the colours of the spectrum as shown in the images below and the computer will display the temperature. As the sensor moves past the red of the visible spectrum it registers a temperature increase and this is displayed on the computer screen.

image   image

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