Exp - 36th International Physics Olympiad. Salamanca...

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36 th International Physics Olympiad. Salamanca (España) 2005 Exp. Page 1 of 11 R.S.E.F. PLANCK’S CONSTANT IN THE LIGHT OF AN INCANDESCENT LAMP In 1900 Planck introduced the hypothesis that light is emitted by matter in the form of quanta of energy h ν . In 1905 Einstein extended this idea proposing that once emitted, the energy quantum remains intact as a quantum of light (that later received the name photon). Ordinary light is composed of an enormous number of photons on each wave front. They remain masked in the wave, just as individual atoms are in bulk matter, but h – the Planck’s constant – reveals their presence. The purpose of this experiment is to measure Planck's constant. A body not only emits, it can also absorb radiation arriving from outside. Black body is the name given to a body that can absorb all radiation incident upon it, for any wavelength. It is a full radiator. Referring to electromagnetic radiation, black bodies absorb everything, reflect nothing, and emit everything. Real bodies are not completely black; the ratio between the energy emitted by a body and the one that would be emitted by a black body at the same temperature, is called emissivity, ε , usually depending on the wavelength. Planck found that the power density radiated by a body at absolute temperature T in the form of electromagnetic radiation of wavelength λ can be written as () 1 / 5 1 2 = T c e c u (1) where c 1 and c 2 are constants. In this question we ask you to determine c 2 experimentally, which is proportional to h . For emission at small , far at left of the maxima in Figure F-1, it is permissible to drop the -1 from the denominator of Eq. (1), that reduces to / 5 1 2 T c e c u = (2) The basic elements of this experimental question are sketched in Fig. F-2. The emitter body is the tungsten filament of an incandescent lamp A that emits a wide range of ’s, and whose luminosity can be varied. The test tube B contains a liquid filter that only transmits a thin band of the visible spectrum around a value λ 0 (see Fig. F-3). More information on the filter properties will be found in page 5. Finally, the transmitted radiation falls upon a photo resistor C (also known as LDR, the acronym of Light Dependent Resistor). Some properties of the LDR will be described in page 6. The LDR resistance R depends on its illumination, E, which is proportional to the filament power energy density 0 0 Eu Ru RE γ ⇒∝ where the dimensionless parameter γ is a property of the LDR that will be determined in the experiment. For this setup we finally obtain a relation between the LDR resistance R and the filament temperature T T c e c R 0 2 / 3 = ( 3 ) that we will use in page 6. In this relation c 3 is an unknown proportionality constant. By measuring R as a function on T one can obtain c 2 , the objective of this experimental question.
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This note was uploaded on 11/08/2011 for the course PHYS 0000 taught by Professor Na during the Spring '11 term at Rensselaer Polytechnic Institute.

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Exp - 36th International Physics Olympiad. Salamanca...

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