Quantum theory, the modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles, is one of the most important theories devised in the 20th century. The theory is revolutionary as it replaces classical physics in the description of events at the microscopic level and now the theory provides the foundation for modern physics and chemistry.
The person who formed the basis of the quantum theory was Max Planck. In the 19th century, scientists used laws of classical physics to explain the relationship between matter and energy. Toward the end of the 19th century, various experimental results were obtained that could not be explained by classical physics. One of the failures of classical physics was the inability to explain the observed frequency distribution of radiant energy emitted by a hot blackbody. Classical physics predicts that when a blackbody is heated, the frequencies of the light radiated will take on a continuous range of values from zero to infinity. However, from experimental observations, the frequency distribution reaches a maximum and then falls off to zero as the frequency increases. In 1900, Max Planck announced a theory to explain the observed frequency distribution of blackbody radiation. He suggested that a blackbody atom radiating light of frequency v is restricted to emitting an amount of energy given by hv (where h is the Planck’s constant). Planck called this definite amount of energy a quantum of energy. In classical physics, energy is a continuous variable. In quantum physics, energy is quantized, meaning that energy can take on only certain values.
After Planck announced his theory, Albert Einstein applied the concept of energy quantization to the explanation of the experimental observations in the photoelectric effect. The photoelectric effect is a phenomenon when electrons are ejected from a substance exposed to electromagnetic radiation. According to classical physics, the average energy carried by an ejected electron should increase with the intensity of the incident radiation and not the frequency. However, from experimental observations, the energy of electrons ejected depends on the frequency of the incident radiation. Increasing the intensity of the incident radiation would only increase the amount and not the average energy of the electrons ejected. Also, for every substance irradiated, there is a threshold frequency below which no electrons are ejected irrespective of the light intensity. In 1905, Einstein explained the photoelectric effect by extending Planck’s concept of energy quantization to electromagnetic radiation. He proposed that besides having wavelike properties, electromagnetic radiation can be considered to consist of individual quanta, called photons, which interact with the electrons in the substance like discrete particles. For a given frequency v of the incident radiation, each photon carries a definite amount of energy given by hv, where h is the Planck’s constant. The threshold frequency is explained by the different nature of the materials. For each material there is a certain minimum energy, called the work function F, necessary to liberate an electron. Thus the threshold frequency, v0, corresponds to a minimum energy packet, hv0 (=F), required to liberate the electron.
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