Why do photons travel 1

Sexl Physik 7, textbook

Radiation pressure When a stream of photons hits a body and is absorbed, it exerts a pressure p Str = P Str / (c · A), where P Str is the power of the incident radiation on surface A. The radiation pressure of sunlight is impressively noticeable in cometary tails (92.1), it affects satellites and is a source of error, especially with GPS satellites, it sends the particles of the solar wind on a journey through the solar system. In terrestrial applications, the radiation pressure from lasers helps to hold atoms, molecules and also biological cells in optical traps. Derivation of the formula: If N photons with the momentum h · f / c fall on the surface A in the time span t and are absorbed, they transfer the amount of energy h · f · N = P Str · t and at the same time an impulse (impulse) F · T = N · h · f / c. The radiation pressure p Str is therefore p Str = F / A = (1 / c) h · f · N / (t · A) = P Str / (c · A).  The Compton effect When investigating the properties of X-rays, the American physicist A RTHUR H. C OMPTON (1892–1962) discovered a strange effect in 1922. B. on a graphite block, its frequency is reduced. This effect cannot be explained with the wave theory of light. It predicts that the electrons in the graphite block will be excited to vibrate by the incident radiation and thus emit radiation with the same frequency. (Graphite is electrically conductive and therefore contains conduction electrons that can move freely inside the graphite block.) Compton was able to interpret the frequency reduction using the photon model. During the scattering, the incident photons collide elastically with almost free electrons in the graphite and give off part of their energy E = h · f during the collision. Therefore the scattered photons have a reduced energy E '= h · f', which can be calculated using the conservation laws for energy and momentum. (92.2) This confirms the prediction p = h / λ for the momentum of the photon. Compton effect, X-ray spectrum and other confirmations of the photon model contrasted the wave theory of light with a particle theory of light. Do the two theories contradict or complement each other? Light in everyday life - where is the particle aspect? Let's look at the sunlight. The sun shines on the earth with an output of 1,400 W / m 2 (92.3). The maximum intensity at λ = 500nm corresponds to a photon energy of approx. 2.5 eV = 2.5 · 1.6 · 10 −19 J = 4.0 · 10 −19 Ws. With an average photon energy of 2.5 eV, this means a photon flux of around N = 3.5 · 10 21 particles / (m 2 · s). About 3.5 · 10 21 photons hit the earth per square meter and second. The analogy to a gas in which the large number of molecules in a container (6 · 10 23 per mole) makes the pattering of the molecules against the container wall appear as a continuous pressure suggests: Because of the large number of Everyday phenomena appearing photons, light appears to us as a continuous wave. We therefore expect that the particle aspects of light become particularly clear when the number of photons is small. 92.1 Comets are “dirty snowballs” because they largely consist of frozen water, CO, CO 2, H 2 S, CH 3 OH, ... as well as mineral dust and debris. In the vicinity of the sun, the frozen gases evaporate and envelop the comet's nucleus in a gas cloud from which two comet tails emerge. The pale yellow dust tail follows the comet's orbit and is influenced by the radiation pressure from the sun. The blue tail consists of atoms that flow away under the influence of the solar wind (protons and electrons). The pictured comet Hale-Bopp was visible to the naked eye for months in 1997. short-wave incident photon long-wave scattered photon resting electron scattered electron 92.2 Compton effect: An energetic photon transfers energy and momentum to an electron in an elastic collision. 1400 W / m 2 92.3 About 4.5 · 10 35 photons with an average energy of 2.5 eV (mainly visible light) hit the earth (R E ≈ 6400 km). The photons are absorbed and heat the earth's surface. The earth radiates this energy as long-wave heat radiation. 92 QUANTUM PHYSICS For testing purposes only - property of publisher öbv

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