Teachers Guide to the Infrared

Teachers Guide to the Infrared

What is infrared light? How does it compare to visible light?

Our eyes are detectors which are designed to detect visible light waves (or visible radiation). There are forms of light (or radiation) which we cannot see. Actually we can only see a very small part of the entire range of radiation called the electromagnetic spectrum .

The electromagnetic spectrum includes gamma rays, X-rays, ultraviolet, visible, infrared, microwaves, and radio waves. The only difference between these different types of radiation is their wavelength or frequency. Wavelength increases and frequency (as well as energy and temperature) decreases from gamma rays to radio waves. All of these forms of radiation travel at the speed of light (186,000 miles or 300,000,000 meters per second in a vacuum).

Infrared 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 can be used as a way to measure the heat radiated by an object. 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 they produce. Any object which has a temperature i.e. anything above absolute zero (-459.67 degrees Fahrenheit or -273.15 degrees Celsius or 0 degrees Kelvin), radiates in the infrared. Absolute zero is the temperature at which all atomic and molecular motion ceases. 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 which we feel as heat. The warmer the object, the more infrared it emits. We experience infrared radiation every day. The heat that we feel from sunlight, 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. We also commonly use infrared rays when we operate a television remote.

Note: To help your students understand that the Sun does indeed put out infrared light, you might want to have them perform The Herschel Experiment, in which they will have the opportunity to discover the existence of infrared light in sunlight for themselves.

How do infrared cameras work?

Thermal infrared imagers are detector and lens combinations that give a visual representation of infrared energy emitted by objects. Thermal infrared images let you see heat and how it is distributed. A thermal infrared camera detects infrared energy and converts it into an electronic signal, which is then processed to produce a thermal image and perform temperature calculations. Thermal imaging cameras have lenses, just like visible light cameras. But in this case the lens focuses waves from infrared energy onto an infrared sensor array. Thousands of sensors on the array convert the infrared energy into electrical signals, which are then converted into an image.

What specifically do infrared images reveal?

Infrared is a type of light that we cannot see with our eyes. Our eyes can only see what we call visible light. Infrared light brings us special information that we do not get from visible light. It shows us how much heat something has and gives us information about an object's temperature. Everything has some heat and puts out infrared light. Even things that we think of as being very cold, like an ice cube, put out some heat. Cold objects just put out less heat than warm objects. The warmer something is the more heat it puts out and the colder something is the less heat it puts out. Hot objects glow more brightly in the infrared because they put out more heat and more infrared light. Cold objects put out less heat or infrared light and appear less bright in the infrared. Anything which has a temperature puts out infrared light. In the infrared images shown in these lesson plans, different colors are used to represent different temperatures. You can find out which temperature a color represents by using the color-temperature scale show to the right of most of the images. The temperatures are in degrees Fahrenheit.

To the left is an infrared image of a metal cup holding a very hot drink. Notice the rings of color showing heat traveling from the liquid through the metal cup. You can see this in the metal spoon as well. To the right is an infrared image of a melting ice cube. Notice the rings of color showing how the melt water warms as it travels away from the cube. Although the ice cube is cold, it still puts out heat, as you can see by matching the color of the ice cube with its temperature.

A visible light picture (left) and an infrared picture (right) of two cups. One cup contains cold water, while the other contains hot water. In the visible light picture we cannot tell, just by looking, which cup is holding cold water and which is holding hot water. In the infrared image, we can clearly "see" the glow from the hot water in the cup to the left and the dark, colder water in the cup to the right. If we had infrared eyes, we could tell if an object was hot or cold without having to touch it.

By using special infrared cameras, we can get a view of the infrared world. These cameras are very useful and have even helped save people's lives. In the infrared, you can "see" in the dark. Even if the Sun is down and the lights are off, the world around us still puts out some heat. The infrared picture to the right shows deer in a forest during a dark night. Notice how we can clearly see the heat from the deer, especially from areas not covered with thick fur like the ears, face and legs. The trees and the ground put out less heat than the deer, but can still be seen through an infrared camera.

Warm-blooded animals, like people, try to keep the same body temperature during both the day and the night. Their body temperatures do not change when it gets dark or cold outside and their heat remains about the same. This makes infrared cameras very useful for finding people who are lost at night or lost at sea. The warm body heat from a person will cause them to glow brightly in the infrared, even in the dark or floating in a cold sea. Police can use infrared cameras to find criminals hiding in the dark and firefighters also use infrared cameras to find the hot spots in a fire.

Infrared cameras are also a good way to study warm-blooded animals at night, and are used to study how animals use fur, feathers and blubber to keep themselves warm. They are also useful for showing the difference between warm and cold-blooded animals. To learn more about warm and cold-blooded animals visit our The Infrared Zoo website.

Above are infrared images of a warm-blooded dog (left) and of a warm-blooded human holding a cold-blooded caterpillar (right). Warm-blooded animals, like the dog shown above, make their own heat. In the infrared picture you can see how the dog's fur keeps some of this heat from escaping, keeping the dog warm. Insects are cold-blooded, which means that they cannot make their own body heat. Instead they take on the temperature of their surroundings. The cold-blooded caterpillar appears very dark (cool) in the infrared compared to the warm-blooded human who is holding it. Notice how the caterpillar is at about the same temperature as the surrounding air.

Another interesting fact about infrared light is that it can travel through thick smoke, dust or fog, and even some materials.

Above is a visible (left) and infrared (right) view of a person's hand inside a black plastic bag. In the visible image, the hand cannot be seen. In the infrared image, however, the heat from the hand can travel through the bag and can be seen by an infrared camera. Infrared light can pass through many materials which visible light cannot pass through. However, the reverse is also true. There are some materials which can pass visible light but not infrared. Notice the man's glasses! Infrared cannot travel through glass. Since this man's body heat cannot travel through his glasses, they appear dark.

Because infrared light can travel through thick smoke and visible light cannot, infrared cameras are used by firefighters to find people and animals in smoke filled buildings. The infrared body heat from people and warm-blooded animals can travel through the smoke and cause them to show up clearly through an infrared camera. Many people and their pets have been saved by firefighters using infrared cameras. Also, because infrared light can travel through thick fog, it is very useful to have infrared cameras on ships and airplanes to help in navigation.

Infrared cameras are also used by satellites in space to measure the temperature of the oceans, to study the Earth's weather during both the day and night, and to study the infrared light from outer space.

To the left is an infrared map of sea surface temperatures, with red being the warmest and purple the coldest. To the right are two images taken by telescopes of a thick area of gas and dust in space where stars are born. Since infrared can travel through thick dust, astronomers can see through thick clouds of dust and gas in space by using infrared telescopes. On the left is a space cloud as seen by a visible light telescope. Notice that we cannot see what lies behind the cloud. In the infrared view (right) we can see through the cloud and find bright, young stars which have just been formed.

To learn more about the everyday applications of infrared imaging visit our web site Seeing Our World in a Different Light

Infrared light is only one of the types of light that we cannot see with our eyes. There are many more, such as X-rays, gamma-rays, ultraviolet light and radio waves. Each of these different types of light brings us new information that we cannot get by using our eyes alone. We are very lucky that we live in a time when we have technology that allows us to "see" all of these types of light.

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