Quantum Properties of Light

Quantum Properties of LightSuch phenomena as the photoelectric effect, light pressure and some others make us perceive the world differently. Thus we consider a luminous flux to be the flux of elementary particles - photons (light quanta). One of the characteristics of a photon is its energy. A monochromatic luminous flux consists of photons with the same energy. There are two types of the photoelectric effect: external and internal. A phenomenon when a metal body, irradiated with light, emits electrons, is called an external photoeffect. The photoeffect is internal when optically excited electrons stay inside the illuminated substance but do not alter its electrical neutrality. According to Einstein, light with v frequency is emitted not only by separate quanta, but in the form of quanta (photons) it is also distributed in space and absorbed by the substance. In other words, quantum energy absorbed by electrons in the photoelectric effect, is spent to make and transmit the kinetic energy to an electron after it has left the substance. This formula is called the Einstein equation for the photoelectric effect. The photoelectric effect is observed only when metal is irradiated with light with a frequency greater than or equal to the critical frequency, called the photoelectric threshold. I.e. the red edge corresponds to photon energy equal to the work function of metal. The velocity of electrons and hence the kinetic energy is zero. The hypothesis of the light pressure was first made by J. Kepler in the 17th century to explain the way the comet tails deviate from the sun. When the light falls on bodies, it puts pressure on them. The pressure depends on the light intensity and surface reflectivity of a body. Experimentally, light pressure on solids was first studied by P.N. Lebedev in 1899. In Lebedev's experiments torsion balance beams with disks-wings attached to theme were suspended in an evacuated glass vessel on a thin silver thread. The wings were irradiated. They were made of various metals and mica with identical opposite surfaces. Sequentially irradiating the front and back surfaces of wings of varying thickness, Lebedev could neutralize the residual effect of radiometric forces and get a satisfactory (with am margin of error of ± 20%) agreement with Maxwell's theory of light pressure. In 1907-1910 Lebedev performed even more subtle experiments on light pressure on gases, and the results complied with the theory too. The formula of light pressure is as follows: Light pressure plays a particularly important role in two ranges of the phenomena that are opposite in scale - in astronomical phenomena and in atomic phenomena. In astrophysics, the pressure of light, along with gas pressure ensures stability of stars as it opposes the forces of gravitational compression. Atomic effects of light pressure include "light output", experienced by an excited atom emitting a photon.

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Science & Technology

Tags:

• light
• optics
• physics