Infrared: I am invisible, how do humans find me?

Eyes are our main organ for obtaining information from the outside world. By using the light received by the human eyes, we can know what the outside world looks like.

However, the human eye's ability to perceive electromagnetic waves is limited. The electromagnetic waves that the eye can observe are only a small part of the electromagnetic waves that exist in nature. We call it "visible light", and the other part is called "invisible light". Infrared is the first invisible light discovered by humans. Its appearance has opened people's eyes. The world under the invisible light field of view is extremely wonderful.

Infrared rays can directly reveal the surface temperature of an object. In fact, all objects are made up of a large number of tiny particles such as molecules or atoms. These tiny particles are constantly vibrating and emitting electromagnetic waves, that is, light. The higher the temperature of an object, the stronger the vibration, and the composition of the light will change accordingly. Heating and melting iron can emit orange-red light.

Iron does not emit light at room temperature. Now we know that the light emitted by iron at room temperature is invisible light that cannot be sensed by the human eye. How can we detect it? The story starts with a piece of glass and a thermometer.

Spectroscopic prism plus thermometer, infrared rays appear

The most famous analysis and understanding of light is Newton's spectroscopic experiment. As a senior optics enthusiast and an important founder of the discipline of optics, Newton let sunlight enter the room through a small hole in a dark room. After the light beam passed through a prism, it was divided into different colors of light. When the different colors of light passed through another prism, the color no longer changed.

Therefore, Newton believed that white light is a mixture of seven colors of light. In order to avoid this phenomenon in telescopes, Newton also invented a reflecting telescope. Most of the large astronomical telescopes in the world today also use reflecting telescopes, which allow light to reflect on the mirror surface instead of entering the glass, thus avoiding the light being divided into different colors and resulting in a decrease in resolution.

At this point, almost everyone thought that Newton's explanation of the composition of light was already very complete. But there was one person who thought differently, and that person was Herschel.

Herschel's father was an astronomer. Naturally, he also liked to observe the stars and became an optical researcher. He made many important contributions to astronomy and photography. He named the satellites of Saturn and Uranus, improved the photographic process, and coined the word "photography".

How did Herschel discover infrared? People have long known that fire emits invisible heat. In 1681, the experimentalist Marriott discovered that when sunlight passes through glass, the skin feels different heat than when it is directly exposed to sunlight. Herschel combined Newton's light dispersion experiment with Marriott's glass experiment to build his own experimental device, and thus discovered the existence of infrared.

(Image source: self-made by the author)

William Herschel placed a prism in a dark room, allowing sunlight to pass through the prism and diverge into different colors of light. Herschel placed thermometers at the positions of different colors of light, and he also placed thermometers in areas outside the red light where the eyes could see no light. After a while, he found that the temperature of the thermometer placed outside the red light rose the fastest. From this, he inferred that there must be some kind of light outside the red light that caused the temperature of the thermometer to rise. He named it heat ray, and because this light was discovered in the range outside the red light, it was also called infrared.

Infrared rays are closely related to our daily life

How useful is infrared? It allows us to know the temperature of an object without touching it.

Even in the dark, infrared imaging equipment can still be used normally because all objects emit infrared rays, which is very useful in rescue, military and other fields. Infrared imaging is very important in astronomical observations. Modern scientists observe the infrared characteristics of matter in the universe. Compared with visible light, infrared rays can penetrate space dust and transmit more information about extraterrestrial galaxies.

(JWST telescope in space, used for infrared imaging, source: wikipedia)

We have said before that since glass can block a considerable amount of infrared rays, why is the interior of a car exposed to the sun in summer still very hot? Although the car windows block the infrared rays from outside from entering the car, they also prevent the infrared rays emitted by objects inside the car from escaping out. The infrared rays are constantly reflected and gathered in the car, and the temperature inside the car becomes higher and higher.

The door to a new world of invisible light is now open

Besides red light, there is infrared light. What about besides violet light? Yes, there is indeed ultraviolet light, and the discovery of ultraviolet light was also the result of scientists conducting an experiment similar to Herschel's, except that the thermometer was replaced with a photographic film. The film outside the violet light will still turn black, indicating that ultraviolet light has irradiated the film.

People's understanding of light is becoming more comprehensive, and various optical instruments have emerged in our lives, such as infrared heating, ultraviolet sterilization, optical detection, weather forecasting, etc. With the help of various invisible lights, our understanding of the world has become more comprehensive.

References:

[1] Ball SRS, Harkness W, Herschel SJ, et al.Essays in Astronomy[M].D. Appleton & Company,1900.

[2] Evans DS , Deeming TJ , Evans BH ,et al.Herschel at the Cape; diaries and correspondence of Sir John Herschel, 1834-1838[J].Physics Today, 1970, 23(11).DOI:10.1063/1.3021829.

Author: Wu Qing, postgraduate student at Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences

Reviewer: Li Ming, Researcher, Institute of High Energy Physics, Chinese Academy of Sciences

The article is produced by Science Popularization China-Creation Cultivation Program. Please indicate the source when reprinting.