Infrared

Some light is invisible.

We cannot see it with our eyes. It is like the heat from a fire.

A man named William Herschel found this light. He used a tool to measure heat. He found light that was not red.

This light carries energy. The sun sends much of it to Earth. It helps keep our world warm.

Special cameras can see this light. They can see heat from a person. This helps people see in the dark.

Scientists use it to look at space. It helps them see through dusty clouds. It is a very useful tool.

Some light is invisible to our eyes. We call this infrared light.

In 1800, a man named William Herschel found it. He used a thermometer to measure heat. He saw that light past red was still warm.

Infrared light is a type of electromagnetic radiation. This is a way energy moves through space. Most of the energy from the Sun reaches Earth as infrared. It helps set the climate of our world.

We can use special tools to see it. Thermal cameras detect heat. They can see the warmth of a human body.

These cameras help firefighters. They can also find heat leaks in buildings.

Scientists use infrared to study space. It lets telescopes see through thick, dusty clouds. This helps us find new planets.

Other tools use near-infrared light. This is light close to what we can see. Night-vision goggles use this to see in the dark.

Infrared light is a special kind of energy called electromagnetic radiation. It is invisible to our eyes, but it is all around us. This light has wavelengths that are longer than red light. However, its wavelengths are still shorter than microwaves. Most objects near room temperature give off energy in this band. This is known as black-body radiation. Even though we cannot see it, this light carries energy and momentum.

This energy works in a very specific way. It is emitted or absorbed when molecules change their movements. Specifically, it changes how a molecule rotates or vibrates. This happens because of a change in the dipole moment. Scientists use a tool called infrared spectroscopy to study this. This tool looks at how light is absorbed or sent through a sample. By studying these energy states, we can learn a lot about molecules.

People have been studying this invisible light for a long time. In 1681, a man named Edme Mariotte did an experiment. He showed that glass blocks radiant heat even if sunlight passes through it. Later, in 1800, the astronomer Sir William Herschel made a huge discovery. He used a thermometer to find invisible radiation. He saw it was lower in energy than red light. His work showed that more than half of the Sun's energy arrives as infrared.

There are many different types of infrared light. Near-infrared is part of the solar spectrum from the Sun. Long-wavelength infrared is often called thermal infrared. This is the heat we feel from things on Earth. For example, humans at normal body temperature radiate at 10 micrometers. Scientists often divide the spectrum into sections like NIR, SWIR, and LWIR. These names help experts know which part of the light they are using.

We use infrared technology in many parts of our lives. Thermal cameras help firefighters and detect heat loss in buildings. In space, infrared telescopes see through dusty clouds to find planets. Some military tools use it for night vision or tracking. Even doctors can use it to observe blood flow in the skin. It is a tool that helps us see the hidden parts of our world.

Infrared radiation, often called infrared light, is a form of electromagnetic radiation (EMR). It consists of waves that are longer than visible red light but shorter than microwaves. Because its wavelengths exceed the limits of human vision, infrared is invisible to us. However, it is a vital part of the energy that moves through our universe. Most objects at room temperature emit radiation within this specific infrared band. This process is known as black-body radiation.

The way infrared interacts with matter depends on the movement of molecules. Infrared radiation is emitted or absorbed when molecules undergo changes in their rotational-vibrational movements. This process occurs because the radiation excites vibrational modes in a molecule. This excitation happens through a change in the dipole moment, which is a measure of the electrical distribution within the molecule. Scientists use a technique called infrared spectroscopy to study these energy states. By examining how photons are absorbed or transmitted, they can identify the specific properties of different molecules.

Researchers often divide the infrared spectrum into several distinct regions based on wavelength. Near-infrared (NIR) is the region closest to the visible spectrum, ranging from approximately 0.75 to 1.4 micrometers. Short-wavelength infrared (SWIR) follows, covering about 1.4 to 3 micrometers. Mid-wavelength infrared (MWIR) is often called thermal infrared and spans from 3 to 8 micrometers. This specific band is used by heat-seeking missiles to track the exhaust plumes of aircraft. Finally, long-wavelength infrared (LWIR) covers 8 to 15 micrometers. This is the "thermal imaging" region used to detect the heat of objects like the human body.

Our understanding of this invisible energy grew through centuries of scientific inquiry. In 1681, the experimenter Edme Mariotte discovered that glass blocks radiant heat even though sunlight passes through it. Later, in 1800, the astronomer Sir William Herschel conducted a landmark experiment. He used a thermometer to detect radiation that was invisible to the eye. He found that this radiation existed at a lower energy level than red light. Herschel’s work eventually helped reveal that slightly more than half of the energy from the Sun arrives on Earth as infrared radiation.

The energy provided by the Sun is immense and highly structured. At sea level, sunlight provides an irradiance of just over 1 kilowatt per square meter. Of this total energy, 527 watts is infrared radiation, while 445 watts is visible light and 32 watts is ultraviolet. Most of this solar infrared is categorized as near-infrared, with wavelengths shorter than 4 micrometers. On Earth, the balance between absorbed and emitted infrared radiation is a critical factor in regulating the planet's climate. This relationship is a central part of the greenhouse effect.

Modern technology utilizes infrared in many surprising ways. In astronomy, sensor-equipped telescopes use infrared to penetrate dusty molecular clouds in space. This allows scientists to detect distant planets and view highly red-shifted objects from the early universe.

In medicine and industry, thermal-imaging cameras detect heat loss in insulated systems or changes in blood flow in the skin. Even in art, infrared reflectography can be used to see hidden layers of a painting, such as those in the Mona Lisa.

Military applications also rely on infrared for surveillance, target acquisition, and night-vision devices.

Infrared radiation connects many different scientific fields. It bridges the gap between visible light and the microwave portion of the electromagnetic spectrum. The study of its absorption helps chemists understand molecular symmetry and structure. In environmental science, infrared monitoring helps track weather patterns and climate changes. Even in telecommunications, specific infrared windows are used for long-distance data transmission. From the smallest molecule to the largest galaxy, infrared radiation provides a window into the mechanics of the natural world.