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Answer to Question #34352 in Quantum Mechanics for soni sharma

Question #34352
please sir explain me planck theory of radiation and photoelectric effect in detail..........??
Expert's answer
Thermal radiation is emitted by an object at any temperature above absolute zero. The classical perspective on thermal radiation presents a model of charged particles accelerating (vibrating) and emitting electromagnetic waves similarly to the charges of a broadcast antenna. The breakdown in the classical model occurred in that the distribution of observed radiation emitted by a black body at short wave lengths, a contradiction known as the ultraviolet catastrophe. Planck's theory contained two breakthrough postulates which were the starting points of quantum theory, firstly, that the energy of vibrating molecules were quantized, possessing discrete states of energy or quantum states, and secondly, that the light energy absorbed or emitted was quantized. That light consists of discrete units of energy, quanta, or photons.


Here is Planck's famous expression of photon energy: E = hf
nder the right circumstances light can be used to push electrons, freeing them from the surface of a solid. This process is called the photoelectric effect (or photoelectric emission orphotoemission), a material that can exhibit this phenomena is said to be photoemissive, and the ejected electrons are called photoelectrons; but there is nothing that would distinguish them from other electrons. All electrons are identical to one another in mass, charge, spin, and magnetic moment.
The photoelectric effect was first observed in 1887 by Heinrich Hertz (1857-1894) during experiments with a spark-gap generator — the earliest form of radio receiver. In these experiments, a spark is generated between two small metal spheres in the transmitter to induce a similar spark to jump between two different metal spheres in the receiver. Compared to later radio devices, the spark-gap generator was notoriously difficult to work with. The air gap would often have to be smaller than a millimeter for a the receiver to consistently reproduce the spark of the transmitter. Hertz found that he could increase the sensitivity of his spark-gap device by illuminating it with visible or ultraviolet light. Later studies by J. J. Thomson (1856-1940) showed that this increased sensitivity was the result of light pushing on electrons (which he discovered in 1897).
While this is interesting, it is hardly amazing. All forms of electromagnetic radiation transport energy and it is quite easy to imagine this energy being used to push tiny particles of negative charge free from the surface of a metal where they are not all that strongly confined in the first place. The era of modern physics is one of completely unexpected and inexplicable discoveries, however. Subsequent investigations into the photoelectric effect yielded results that did not fit with the classical theory of electromagnetic radiation. When it interacted with electrons, light just didn't behave like it was supposed to. Repairing this tear in theory required more than just a patch. It meant rebuilding a large portion of physics from the ground up.

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