Atlearner-
April 19, 2023

September 03, 2023

In quantum theory, we encounter phenomena or truths that are beyond our ordinary experience, and that seem unreal at first sight. Even the scientists who laid the foundations of quantum theory were surprised by some of the conclusions reached through theoretical analysis. However, all of the experiments that have been conducted to test this theory have strengthened the foundation of this theory.

Quantum theory explains certain phenomena, particularly those observed at the atomic and subatomic scales. Many physicists such as Max Planck, Albert Einstein, Niels Bohr, Werner Heisenberg, and Erwin Schrödinger played very important roles in shaping this revolutionary theory.

In 1900 AD scientist Max Planck first introduced this theory. Despite the incredible success of the wave theory of light, it was not possible to explain some phenomena like blackbody radiation, the photoelectric effect, atomic spectra, etc.

Finally, Planck developed quantum theory while explaining the spectrum of black body radiation. According to him, radiation consists of energy packets, he called these packets quanta. Later Einstein introduced the concept of the photon particle which is extremely helpful to describe the photoelectric effect. Further, this theory was firmly established by Werner Heisenberg, and Erwin Schrödinger.

A quantum represents the most minimal, distinct unit of a particular phenomenon. For example, a photon stands as a quantum of light, while an electron serves as a quantum of electricity. The term "quantum" originates from Latin, signifying "a quantity" or "to what extent?" When an entity is quantifiable, it possesses the capacity to be measured.

In everyday life, certain things come in fixed amounts that are connected to a basic unit. This basic unit is called a "quantum. For example, currency is quantized and currently, its quantum in the Indian economy is 50 paisa. Obviously, it is possible to give someone 1 rupee 50 paise, or 150 paise, but giving 150.5 paise is not possible. Similarly, in radiation photon is also referred to as a quantum of energy.

As mentioned earlier, the photoelectric effect cannot be explained with the help of wave theory. For this reason, in 1905, Einstein expanded Planck's quantum theory by introducing the concept of photon particles and explained the photoelectric effect.

This particle theory of radiation is now known as quantum theory. The essence of this theory is that electromagnetic radiation is not a wave, but a stream of particles called **photons**.

Now if we talk about the properties of photon then they are as follows

(i). Every photon particle is neutral.

(ii). Every photon particle travels at the speed of light. In any situation, the speed of the photon does not decrease or increase *i.e.* it is constant. (speed of photon or light, c = 3×10⁻⁸ ms⁻¹)

(iii). Amount of energy carried by a photon, *E = h𝜈*; where *𝜈* = frequency of radiation, *h* = Planck's constant. The greater the number of photon particles in the photon stream, the greater the amount of energy carried, resulting in an increase in the brightness of the radiation.

(iv). Mass of the photon cannot be explained by Newtonian mechanics. The idea that photons have mass is rejected. In simple words, the rest mass of the photon is zero.

(v). According to the theory of relativity, if a particle has rest mass *m0* and momentum *p*, then the energy of the particle,

For photon particle *m0 = 0*,

That is, even though the photon is a massless particle, it has definite momentum.

One of the key concepts of quantum theory is **wave-particle duality**. This means that particles, such as electrons and photons, can behave as both particles and waves. For example, when an electron is observed, it appears as a particle, but when it is not observed, it behaves as a wave. This concept was first proposed by Louis de Broglie in 1924 and was later confirmed by experiments.

Considering electromagnetic radiation as a stream of photon particles in particle theory, phenomena like the photoelectric effect, black body radiation, atomic spectra, etc. can be explained. However, this theory cannot explain light phenomena such as interference, diffraction, scattering, dispersion, polarisation, etc.

On the other hand, the wave theory of radiation can easily explain phenomena such as interference, diffraction, scattering, dispersion, and polarisation. Therefore, according to modern theory, radiation (light) sometimes behaves like waves and sometimes like particles.

That is, there are two forms of radiation - wave form and particle form. So wave theory and particle theory are not contradictory, but complement each other like the reverse side of the same coin. This is known as **wave-particle duality**, which is deeply embedded into the foundations of quantum mechanics today.

Another key concept of quantum theory is superposition. This means that a particle can exist in multiple states at the same time. For example, an electron can be in two different energy levels simultaneously. Mathematically, it's when an equation has more than one possible answer.

Superposition means that an object can be in multiple states at once, like ripples on a pond created by two waves coming together. This concept is important in the development of quantum computers, which can perform calculations using superposition.

The Uncertainty Principle is also a very important concept in quantum mechanics. In 1927, It was formulated by German physicist Werner Heisenberg.

This concept says you can't precisely know two things about an object at the same time, like its position and speed. If you know one very precisely, the other becomes uncertain. The Uncertainty Principle tells us that in the tiny world of atoms and particles, we can't know everything at once.

Imagine trying to measure a particle's position and speed. The more accurately we measure one thing, like where it is, the less accurately we can know another thing, like how fast it's moving. This strange rule shows us the limits of our knowledge in the fascinating realm of quantum physics.

Quantum theory also includes the concept of quantum entanglement. This happens when two particles become linked in such a way that the state of one particle is dependent on the state of the other particle, even if they are separated by a long distance.

In this situation knowing about one particle tells you something about the others, even if they're far apart. It's like they act as a single system. This concept has important implications for cryptography and communication, as it could allow for secure communication over long distances.

One of the most famous experiments in quantum theory is the **double-slit experiment**. This experiment involves firing electrons at a barrier with two slits. When the electrons pass through the slits, they interfere with each other, creating an interference pattern on a detector screen.

This experiment demonstrates the wave-like behavior of particles and has important implications for our understanding of the nature of reality.

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- Electromagnetic Waves
- Electromagnetic Induction and Alternating Currents
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