Every day, the rising and setting sun that illuminates the darkness that has been removed. Since ancient times, humans have been curious about the essence of this light. What is light? When you look at light from the perspective of particle-wave duality, how was it actually created? Until the 17th century, scholars fiercely debated whether light was a particle or a wave. However, at this time, a genius scientist emerged. The father of Optics, Isaac Newton. At that time, Isaac Newton, who was considered one of the greatest geniuses of humanity, used a prism to separate light into red, orange, yellow, green, blue, indigo, and violet, and conducted experiments to combine the various colors of light to create white light, proving that light is a composite entity. Furthermore, he assumed light to be composed of particles and explained the reflection, refraction, and dispersion of light by a prism. At that time, Isaac Newton was almost unreasonably pronounced as the only pronoun for scientists.

During the era when people believed in and respected Newton, his theory that light consists of particles was widely accepted. However, scientist Christian Huygens, who did not agree with Newton's particle theory of light, stated, If light is made up of particles, when light hits light, it should bounce off. It should scatter. However, no matter how much light you pile up against light, it does not spread in all directions. This means that light is fundamentally a wave. Not only Huygens but many other scientists continued to argue that light is a wave. Despite this, many did not believe them because Isaac Newton, a physics superstar, had great credibility, so scientists advocating the wave theory were refuted.

There was no opportunity, thanks to Newton's claim, people thought of light as particles for 180 years. Thus, the idea that light is particles dominated. Scientist Thomas Young conducts a double-slit experiment using light. The term slit means a narrow opening. Since there are two narrow openings, it's called a double-slit experiment. Placing a board with two holes and adding another board behind it, let's shine light towards the double slit for the first time. Thomas Young, who thought of light as particles, expected this. Hmm, some light will hit the wall and bounce off, while some will pass through the slits and reach the screen behind the wall. As we expected, the light passing through the holes on the board behind should stick together and create a pattern of lines on the screen.

I expected that, but the result was completely different. Naeun, what's going on? It's not 9 lines, but multiple lines coming out. How can scientists make multiple patterns appear? They start looking for past cases. However, if light is a wave, it is possible. If light is a wave, it can pass through two slits simultaneously from the beginning. If light is a wave, it matches the interference pattern of the double slit. In other words, the conclusion is that light is a wave. In the second experiment, Newton's light particles lost their power, and the wave theory of light became dominant again. In 1862, James Maxwell conducted an experiment to measure the speed of electromagnetic waves generated by electricity and magnetism. In this experiment, Maxwell deduced that the speed of electromagnetic waves is 300,000 km per second. No, it's exactly 300,000 km per second. The speed of electromagnetic waves coincides with the speed of light. Is this the moment where James Clerk Maxwell went crazy?

Maxwell, who was quick-witted, saw through that light in this experiment is a type of electromagnetic wave. So, gentlemen, electromagnetic waves are a dream, a house, or a ghost. Yes, that's right. It has been confirmed that light is indeed transmitted as electromagnetic waves, with the components of the electromagnetic field perpendicular to each other in the form of waves. With the introduction of Maxwell's wave equations, Newton's assertion of light as particles lost its strength, and the theory that light is a wave became much more dominant. On the other hand, Max Planck, who was studying blackbody radiation, realized the fact that the energy of light is intermittent in the process of explaining blackbody radiation. Let's think of it as a racing car. A car can run at speeds of 100km/h and 200km/h, but it is impossible to run at speeds between 200km/h and 250km/h. This fact that energy is intermittent is actually a characteristic of particles.

For example, let's consider the weight of a billiard ball to be 1kg. So, the weight of three billiard balls would be 3kg, as there are three balls. Unless we break the billiard balls, we cannot make 1.5kg or 2.5kg using only billiard balls. In other words, light energy can only exist as a multiple of a specific value. We call this quantization. Energy is not continuous but rather exists in discrete amounts. This sparseness of energy is actually a property of particles. The phenomenon occurring in blackbody radiation is something that cannot happen if light is considered as a wave, but on the contrary, if light is seen as particles, it is possible. Max Planck thought about this and said, For a light to be observed, the smallest unit must be one room leak. However, at that time, the theory that light is a wave was very dominant, and the idea that light is made of particles was unthinkable.

In the scientific community, due to the wave-particle duality of light, Max Planck, even after observing the fact that light behaves like particles, could not easily believe it. Instead, he cautiously expressed light as particles when announcing his groundbreaking research results. On the other hand, scientist Heinrich Hertz, who believed light to be a wave, accidentally discovered the photoelectric effect while conducting cathode ray experiments. The photoelectric effect refers to the phenomenon where electrons are emitted from a metal surface when exposed to light with a frequency higher than a specific threshold frequency. At that time, the scientific community strongly favored the wave theory of light, and they could not explain this photoelectric effect with the wave theory at all. However, at this point, the greatest genius scientist in human history appears to resolve all these debates.

At the time when Albert Einstein introduced the theory of light as particles called photons, which explained the photoelectric effect that wave theory could not, he perfectly explained the photoelectric effect. If you look closely, there are tiny particles, like grains, in light. This is what we call photons. The reason why electrons jump out when light above a certain frequency is shone on a metal is because photons of light collide with electrons. Photons act like particles when interacting with matter, and when they collide with electrons, they transfer their energy to the electrons, causing them to jump out of the metal. In this way, Einstein explained the photoelectric effect, which could not be explained by wave theory, with the theory of light particles. At that time, in the scientific community, claiming the existence of light particles was considered insane, but Einstein completely overturned the traditional theory of light as waves with the photoelectric effect.

He succeeds in winning the Nobel Prize in Physics by conducting an experiment that proves Einstein's particle theory of light. He precisely shoots a beam of X-rays at a blackboard and discovers that light moves like a billiard ball. Now, let's roll the yellow ball towards the red ball. Depending on the angle of impact, the yellow ball will bounce off just as much as the red ball spins. Here, the yellow ball represents a photon, and the red ball represents a black carbon particle. If light were not particles, this phenomenon could never occur. At that time, with the Compton experiment, it was clearly demonstrated that light is composed of particles, and with this experiment alone, he wins the Nobel Prize in Physics in 1927.

So, for hundreds of years, the scientific community has been fiercely debating whether light is a particle or a wave. In the time of Newton, the particle theory of light prevailed. Then, during the era of Thomas Young and Maxwell, the wave theory of light dominated. Once again, with Einstein's discovery of the photoelectric effect, the particle theory of light became superior. After centuries of arguing whether light is a wave or a particle, scientists were at a loss. In the scientific community, the concepts of particles and waves were considered mutually exclusive. However, in experiments where neither theory seemed to fit, sometimes the wave theory had to be correct, and in other experiments, the particle theory had to be true, yielding strange experimental results. At this point, scientists began to accept the duality of light, acknowledging that sometimes it behaves as a wave and other times as a particle.

Where exactly will Einstein's photoelectric effect be used? Let's end it with a movie reference. In the movie Ocean's Twelve, the world's greatest thief, François Toulour, infiltrates a secure room to steal a valuable treasure. This security room is equipped with a special laser that, if touched by a body, triggers the alarm, leading to the failure of the heist. From the thieves' perspective, this laser security device is quite annoying. However, what is the principle of this security laser? The principle of this security laser is precisely the photoelectric effect. According to the photoelectric effect, light energy can be converted into electrical energy and vice versa. Let's take a look at the laser beam. When a current is applied to a semiconductor, - cut off

When electrons move, light energy is emitted. In laser security technology, analyzing the point where the laser touches the body and reflects back with infrared light can distinguish between humans and objects, calculate the time it takes for the light to reflect back, and determine the position and size accurately. Calculations through light are extremely precise. Even in severe conditions like heavy fog or rain, objects can be detected and whether it is a living organism that is moving can also be detected. This laser sensing technology that uses light is thanks to Einstein's photoelectric effect. The reason we can see such great scenes in movies is also due to the photoelectric effect. Furthermore, areas where the photoelectric effect is used include automatically turning on and off lights, automatic doors, scanners, navigation systems, electronic calculators, artificial satellites, etc. Oh, today's lighting article was published. We really covered a lot of English today, right? However, the properties of waves and particles simultaneously.

Is having this all indeed material? Could it possibly be both a wave and a particle at the same time? Would it make sense to say that matter has eyelashes? The content of tonight is the history of quantum mechanics. So far, it has been about movies and YouTuber reviews.

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