It may or may not surprise you to learn that light isn’t simply limited to the light we can see. There is a spectrum of light, which encompasses both visible (what we can see) and non-visible light (what we can’t see, at least not with our eyes). The light spectrum is a name given to all the wavelengths of light ranging from radio waves to gamma rays.
What is Infrared Light?
Infrared light is a form of electromagnetic radiation that has longer wavelengths than light visible to our naked eyes. On the light spectrum it typically falls between visible light and microwaves, ranging from 780nm to 1,000,000nm (1 million nm or 1mm).
How was Infrared Light Discovered?
It was a well-known fact that fires emitted an “invisible” heat, but it wasn’t until 1800, when famous astronomer William Herschel began to experiment that he discovered that there was something beyond what our eyes could see.
He used a prism to refract light from the sun and detected what we now know as infrared beyond the visible red part of the spectrum, which he noticed due to an increase in the temperature recorded on a thermometer. He initially referred to these as “Calorific Rays”. The term “infrared” was not coined until later.
It was three-time president of the Royal Photographic Society, Sir William de Wiveleslie Abney, who coined the term “Infra-red” (initially spelt with a hyphen). Abney was also the first person to utilize infrared for scientific photographic application.
What can Infrared Light be Used For?
Heat
Infrared light provides what we feel as heat, such as heat from the sun. Much of the sun’s energy comes from the infrared frequency range, which heats objects directly without heating the surrounding air.
Humans, for example, as well as animals and plants, naturally absorb the heat from the infrared rays of the sun. Sunlight on the Earth’s surface is around 52 – 55 % infrared (above 700nm), 42 – 43 % visible light (400 – 700nm), and 3 – 5 % ultraviolet (below 400nm).
Scientific Images
It can be used in scientific applications, a recent notable example being the James Webb telescope. The James Webb Space telescope utilizes supercooled infrared photodetectors to see through clouds of dust in space and into nebulas where stars are born, which traditional cameras and photographic techniques could not permeate.
The Miri instruments of the James Webb telescope operate in the mid infrared region at wavelengths of around 5,000nm to 28,000nm. This is not the same infrared region which is utilized for fiber optics as we would have very high losses.
In addition to being able to see through this dust, the James Webb telescope has succeeded in capturing images from objects emitting light as long as 13 billion years ago. The telescope was only able to capture these images using infrared photodetectors as the light had travelled so far and for so long that it had begun to deteriorate and go through a “redshift”, making it only visible in the infrared part of the light spectrum.
Find out more about the James Webb telescope here:
Image from James Webb telescope via Nasa
Communication
In fiber optics, infrared light is used to transmit data as the wavelengths have significantly lower absorption (attenuation) in standard telecommunication glass optical fibers.
This allows the longest distance transmission of data from point to point with minimal loss of power. The wavelengths that are used to transmit signals along glass optical fibers range between 800nm – 1600nm.
So, without infrared, long-distance communication through glass optical fibers would not be possible!
