Light (or radiation) is made up of vibrating waves of electrical and magnetic fields. This is where the term electromagnetic radiation comes from. Electromagnetic radiation travels in waves which have different wavelengths, energies and frequencies. The wavelength is the distance between individual waves (e.g. from one peak to another). The frequency is the number of waves which pass a point in space each second.
Light waves are waves of energy and the amount of energy in a wave is proportional to its frequency. Wavelength increases, while frequency and energy decreases as we go from gamma rays to radio waves. All electromagnetic radiation travels at the speed of light (186,000 miles or 300,000,000 meters per second in a vacuum).
When we look at the world around us we are seeing visible light waves (or visible radiation). However, there are many other forms of radiation that we cannot see with our eyes. These types include gamma rays, x-rays, ultraviolet, infrared, microwaves and radio waves. Together with visible light, all these types of radiation make up what we call the electromagnetic spectrum - the complete spectrum of radiation.
Infrared radiation lies between the visible and microwave portions of the electromagnetic spectrum. Infrared waves have wavelengths longer than visible and shorter than microwaves, and have frequencies which are lower than visible and higher than microwaves.
Infrared is broken into three categories: near, mid and far-infrared. Nearinfrared refers to the part of the infrared spectrum that is closest to visible light and farinfrared refers to the part that is closer to the microwave region. Mid-infrared is the region between these two.
The primary source of infrared radiation is heat or thermal radiation. This is the radiation produced by the motion of atoms and molecules in an object. The higher the temperature, the more the atoms and molecules move and the more infrared radiation they produce.
Any object which has a temperature i.e. anything above absolute zero (the temperature at which all atomic and molecular motion ceases which occurs at -459.67 degrees Fahrenheit or -273.15 degrees Celsius or 0 degrees Kelvin), radiates in the infrared. Even objects that we think of as being very cold, such as an ice cube, emit infrared.
When an object is not quite hot enough to radiate visible light, it will emit most of its energy in the infrared. For example, hot charcoal may not give off light but it does emit infrared radiation which we feel as heat. The warmer the object, the more infrared radiation it emits.
Humans, at normal body temperature, radiate most strongly in the infrared at a wavelength of about 10 microns (A micron is the term commonly used in astronomy for a micrometer or one millionth of a meter). This is the case for most warm-blooded animals. Cold-blooded animals, such as reptiles, will take on the temperature of their environment and are more difficult to detect in the infrared, unless compared to a cooler or warmer background.
Some animals can "see" in the infrared. For example, snakes in the pit viper family (e.g. rattlesnakes) have sensory "pits," which are used to image infrared light. This allows the snake to detect warm blooded animals (even in dark burrows) by imaging the infrared heat that they radiate. Snakes with 2 sensory pits are thought to have some depth perception in the infrared.
We experience infrared radiation every day. We also commonly use infrared rays when we operate a television remote. The heat that we feel from a fire, a radiator or a warm sidewalk is infrared. Although our eyes cannot see it, the nerves in our skin can feel it as heat. The temperature-sensitive nerve endings in your skin can detect the difference between your inside body temperature and your outside skin temperature.
In the year 1800, a German astronomer living in England, Sir Frederick William Herschel (1738-1822), discovered the infrared. Herschel thought that the different colors of the visible spectrum might contain different levels of heat, so he devised a clever experiment to investigate his hypothesis. During this experiment he measured the temperature of the individual colors of visible radiation to find that the temperature of the colors increased from the violet to the red part of the visible spectrum. After noticing this pattern, Herschel decided to measure the temperature just beyond the red portion of the spectrum in a region apparently devoid of light. To his surprise, he found that this region had the highest temperature of all.
What Sir William had discovered was a form of light (or radiation) beyond red light which he called "calorific rays". These "calorific rays" were later renamed infrared rays or infrared radiation (the prefix infra means 'below'). Herschel's experiment was important not only because it led to the discovery of infrared, but also because it was the first time that someone showed that there were forms of light that we cannot see with our eyes.
Herschel's original prism and mirror are on display at the National Museum of Science and Industry in London, England.
Last Updated: 16 February 2011