khí quyển sao p2

Chương này sẽ tóm tắt một số các công thức và định lý liên quan với đenbức xạ. Một điểm nhỏ của phong cách khi từ "đen" được sử dụng như một tính từ, nó làthường được viết như một từ unhyphenated duy nhất, như trong "bức xạ đen", trong khi đó khi"cơ thể" được sử dụng như một danh từ và "đen" như một tính từ, hai từ riêng biệt được sử dụng. Như vậy, một màu đencơ thể phát ra bức xạ đen. Mặt trời phát ra năng lượng rất gần như một màu đencơ thể. Bức xạ...
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khí quyển sao p2 1 CHAPTER 2 BLACKBODY RADIATION2.1 Introduction.This chapter briefly summarizes some of the formulas and theorems associated with blackbodyradiation. A small point of style is that when the word blackbody is used as an adjective, it isusually written as a single unhyphenated word, as in blackbody radiation; whereas whenbody is used as a noun and black as an adjective, two separate words are used. Thus a blackbody emits blackbody radiation. The Sun radiates energy only very approximately like a blackbody. The radiation from the Sun is only very approximately blackbody radiation.2.2 Absorptance, and the Definition of a Black Body.If a body is irradiated with radiation of wavelength λ, and a fraction a (λ) of that radiation isabsorbed, the remainder being either reflected or transmitted, a (λ) is called the absorptance atwavelength λ . Note that λ is written in parentheses, to mean at wavelength λ , not as asubscript, which would mean per unit wavelength interval. The fractions of the radiationreflected and transmitted are, respectively, the reflectance and the transmittance. The sum of theabsorptance, reflectance and transmittance is unity, unless you can think of anything else thatmight happen to the radiation.A body for which a (λ) = 1 for all wavelengths is a black body.A body for which a has the same value for all wavelengths, but less than unity, is a grey body.(Caution: We may meet the word absorbance later. It is not the same as absorptance.)2.3 Radiation within a cavity enclosure.Consider two cavities at the same temperature. Well suppose that the two cavities can beconnected by a door that can be opened or closed to allow or to deny the passage of radiationbetween the cavities. Well suppose that the walls of one cavity are bright and shiny with anabsorptance close to zero, and the walls of the other cavity are dull and black with anabsorptance close to unity. Well also suppose that, because of the difference in nature of thewalls of the two cavities, the radiation density in one is greater than in the other. Let us open thedoor for a moment. Radiation will flow in both directions, but there will be a net flow ofradiation from the high-radiation-density cavity to the low-radiation-density cavity. As aconsequence, the temperature of one cavity will rise and the temperature of the other will fall.The (now) hotter cavity can then be used as a source and the (now) colder cavity can be used as asink in order to operate a heat engine whic h can then do external work, such work, for example,to be used for repeatedly opening and closing the door separating the two cavities. We have thus 2constructed a perpetual motion machine that can continue to do work without the expenditure ofenergy.From this absurdity, we can conclude that, despite the difference in nature of the walls of the twocavities (which were initially at the same temperature), the radiation densities within the twocavities must be equal. We deduce the important principle that the radiation density inside anenclosure is determined solely by the temperature and is independent of the nature of the walls ofthe enclosure.2.4 Kirchhoffs LawKirchhoffs law, as well as his studies with Bunsen (who invented the Bunsen burner for thepurpose) showing that every element has its characteristic spectrum, represents one of the mostimportant achievements of mid-nineteenth century physics and chemistry. The principal resultswere published in 1859, the same year as Darwins The Origin of Species, and it has beenclaimed that the publication of Kirchhoffs law was at least as influential in the advance ofscience as the Darwinian theory of evolution. It is therefore distressing that so few people canachieve the triple task of spelling his name, pronouncing it correctly, and properly stating hislaw. Kirchhoff and Bunsen laid the foundations of quantitative and qualitative spectroscopy.Imagine an enclosure filled with radiation at some temperature such that the energy density perunit wavelength interval at wavelength λ is u λ(λ ). Here I have used a subscript andparentheses, according to the convention described in section 1.3, but, to avoid excessivepedantry, I shall henceforth omit the parentheses and write just u λ . Imagine that there is someobject, a football, perhaps, levitating in the middle of the enclosure and consequently beingirradiated from all sides. The irradiance, in fact, per unit wavelength interval, is given byequation 1.17.1 Eλ = uλc /4 2.4.1If the absorptance at wavelength λ is a(λ), the body will absorb energy per unit area per unitwavelength interval at a rate a (λ)Eλ.The bod ...