Article about light

Light is the most important need of our life. It seems impossible to imagine life without light. But what is light? This question is not so simple. Einstein, who contributed to the understanding of the origin of light, was so dissatisfied with the current perception of the reality of light that, near the end of his life, he said:

"Despite all fifty years of conscious effort, I have not been able to find the final answer to the question, 'What is the reality of light?' Of course, everyone today thinks they know the answer to this question, but they are deceiving themselves."

In this article, I will provide a brief overview of the ideas that have been put forward in different periods of human history about the reality of light and then turn to the important question of what is ultimately the basis of the mystery associated with light. But great scientists like Einstein seem unsatisfied. 

The nature of light has been a subject of interest since ancient times. In the beginning the study of light was mainly concerned with vision. For example, the ancient Egyptians believed that light was the seeing activity of their god, Ra. When the eye of Ra (the sun) opens, it is day. And when the eye of Ra is closed, it is night. 

The earliest studies of the nature of light and vision can be traced back to Greek traditions. Greek civilization produced many early concepts about vision through the works of Democrates, Epicurus, Plato, and Aristotle. 

The first theory related to vision was presented by Plato. The central idea of ​​this theory was that just as the other senses, such as touch and taste, perceive objects, vision is caused by light rays coming from the eye that perceive objects. According to this theory, when we open our eyes, light rays come out and collide with different objects. Thus, by 'touching' these rays, we can perceive the distance to different objects, their size, shape and color. Interestingly, this theory of vision perception, which today would be considered very strange, persisted for almost a thousand years. Meanwhile, scientists of the Greek period, such as Euclid and Galen, and philosophers of early Muslim history, such as Al-Kandi, picked up on this theory and advanced it. 

This theory of vision and light was conclusively disproved by Ibn al-Haytham, an Arab scientist, at the beginning of the eleventh century. He proved that contrary to the traditional view of vision, light is not produced by the eye, but by bright objects. 

Abu Ali al-Hasan ibn al-Hasan ibn al-Haytham, known in the West as al-Hazen, is a central figure in the history of science. He was the first person to adopt the concept that a scientific theory is based on experiments. As such, many consider him to be the first scientist in human history. 

Ibn al-Haytham disproved the theory proposed by Plato and other scientists that light emanates from the eye and proved that light emanates from sources of light. He proved this by a simple experiment. They sent light through a hole into a dark room through two lanterns placed at different heights, so light appeared at two points on the wall of the room. This light was due to the rays that passed through the holes in the wall from each lantern. When they covered a lantern, the light associated with that lantern disappeared. From this he concluded that light does not originate from the human eye but is produced by objects such as lanterns and travels in straight lines from these objects. 

Attempts to understand the nature of light began as early as the seventeenth century. At the end of this century, Isaac Newton proposed that light consists of tiny particles. 

Newton supported the material theory of light by thinking about the nature of light. According to him, light is made up of extremely small particles, while ordinary matter is made up of large particles. He hypothesized that through a kind of chemical change, light and matter particles are transformed into each other. In Newton's words,

"Are not large bodies and light interchangeable, and may not bodies derive much of their activity from the particles of light which enter their composition?"

It is surprising that Newton advocated a theory of light, according to which light consists of material particles. While there was evidence that supported the wave behavior of light. 

By the beginning of the nineteenth century, Newton's position in the scientific world was so great, especially in the British Isles, that few dared to challenge his theory of light. However, almost a hundred years later, a British scientist, Thomas Young, in 1802 AD, proved through a double slit experiment that light does not consist of particles, but is a weightless wave. 

Young's double-slit experiment was not only decisive in ultimately dispelling Newton's theory of light, but it continued to contribute to our understanding of the nature of light and matter well into the twentieth century. 

To understand the nature of light, we try to understand this historical experiment of Thomas Young. 

Imagine a screen with two tiny holes in it. Rays of light are sent to this screen. These rays pass through these two holes and fall on a wall. Now the question is, what will we see on the wall? 

At every point on the wall, the light passes through the two holes in the screen. We will see a pattern of bright and dark points on the wall. This happens because the waves emanating from these two holes reinforce each other at some points on the wall, making these spots brighter, and cancel each other out at other spots, making these spots darker. This situation is like dropping two pebbles into the still water of a lake. We see a pattern of wave fluctuations. 

Interestingly, if the same experiment is done with particles, this pattern of light and dark cannot be obtained. In this case, some particles will pass through one hole and some through the other hole. The result is that the particles will reach the wall either through one hole or the other. In this case, the process of merging waves will not be seen. 

Thus, the pattern of light and dark symbolizes that light is like a wave. 

Wave properties include wavelength and frequency. Light and sound are examples of waves. 

The classical picture of light was completed by James Clark Maxwell in 1865 AD when he proved that light waves are composed of electric and magnetic waves and travel at a speed of 30,000 kilometers per second. 

At the end of the 19th century, the idea was prominent that all the laws of nature had been discovered and that science had reached its final destination. Maxwell's picture of light came to be considered definitive. 

Then, in December 1900 AD, a revolution occurred that forever changed our understanding of the nature of light, and depicted a Which is beyond common understanding till date. This revolution was associated with the discovery of quantum mechanics. 

In the 1890s, Hendrich Hertz (and later Philip Lennard) observed that when a metal, such as iron or copper, is exposed to light, negatively charged electrons are ejected from the surface of the metal. The model used to explain this observation was that electrons are part of an atom and if given enough energy, which is different for different metals. So they come out of metal. These electrons are called photoelectrons. 

However, it was observed that, for certain colors such as red, no photoelectrons are emitted, no matter how intense the light beam. However, for other colors, such as blue and violet, photoelectrons are emitted, no matter how weak the light beam. For such a light beam, the emission of photoelectrons occurs almost instantaneously, without any delay, after the light is shone on the metal. 

These observations were extremely surprising and could not be explained on the basis of the classical laws of physics at the end of the nineteenth century. For example, how can it be that for some colors of light photoelectrons are not emitted even when the intensity of the light is very high, but for some other colors photoelectrons are emitted even with a weak light beam? And then the most mysterious thing was the instantaneous emission of photoelectrons even whenA very weak beam of light is focused on the metal. 

At that time the picture of light was that it consisted of waves, and if strong enough waves of light were incident on a metal the energy of the photon could be sufficient to eject an electron. It may take some time for these waves to accumulate enough energy, but when this energy is accumulated, it can be supplied to the electrons that cause photoelectron emission. However, this wave picture was unable to explain the observed behavior. There was no reason why certain color waves should be able to eject electrons and other colors could not. 

Einstein explained the photoelectric effect using Max Planck's hypothesis of energy quanta. Einstein hypothesized that light consists of a collection of particles, or quanta, called photons. The amount of energy in each photon depends on the color, and each color is identified by its frequency. For example, red photons with lower frequencies have less energy than blue photons. The colors of the rainbow increase in photon energy from red, orange, yellow, green, blue, indigo, to violet. When one of these photons enters a metal, it transfers all of its energy to an electron. 

Einstein postulated that each photon 'hits' an electron and imparts its energy to the electron. If the photon energy exceeds a certain minimum value for a given metal, the electron is ejected from the metal. This explanation explains why, for a particular metal, a beam of red light cannot eject an electron no matter how intense the light is. Red photons, however large in number, do not have enough energy to eject an electron. On the other hand, a blue photon, even just one, has enough energy to force an electron out. This explains the instantaneous emission of photoelectrons. 

An analogy can explain this behavior. Suppose, we want an unwanted person to be removed from a room. If a large army of six-inch tall Lilliputians were sent for this purpose, they would not succeed. But a single powerful person, like Gulliver, should be enough to get the person out of the room. In Einstein's explanation, low-frequency red photons play the role of Lilliputians and high-frequency blue photons play the role of Gullivers. 

So, in one fell swoop, Einstein's concept of the photon explained the photoelectric effect beautifully. The highlight of this explanation was that it presented a 'particle' picture for light and introduced the concept of 'light quanta' or photons. It was a bold move. 

This was the first time that light was introduced as a collection of weightless particles such as photons. The idea that light consists of photons greatly influenced later developments in the full formulation of quantum theory. 

Einstein very successfully explained the photoelectric effect with the concept of photon. But there was a big difficulty in this explanation. 

Now there was a strange situation about the reality of light. On the one hand, the photoelectric effect could only be explained if light was treated as a particle (photon). If light is considered as a wave, it is impossible to explain the photoelectric effect. On the other hand, Thomas Young's double-slit experiment could only be explained if light was considered as a collection of waves. If light is considered to be a collection of particles, it becomes impossible to explain Thomas Young's experiment. 

Thus, we have a contradictory picture of light: in some experiments it behaves like a wave and in others, it behaves like a particle. 

It is not possible to answer the question whether light is a wave or a particle. Most mysteriously, whether light behaves like a wave or a particle depends entirely on which experiment we decide to do. If Thomas Young's double slit experiment is performed, light is a wave and if photoelectric experiment is performed, the same light becomes a particle. 

Another question complicates the puzzle. If we perform a photoelectric experiment and the light appears as a collection of particles, can we say that the light was composed of particles before the experiment? 

Surprisingly, quantum mechanics does not allow us to say what the origin of light was before we experimented. 

Another question, to which no one has an answer, is at what stage in the experiment light chooses to be either a wave or a particle. 

All these questions have been unanswered for a hundred years. 

Quantum mechanics is a mysterious theory based on the concept of wave-particle duality. What is the reality of light? The final answer to this question may be given by future generations. 

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