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Unformatted text preview: 2/15/11 Quiz QuesOon #1 Which light travels fastest? Astro 109 Lecture 9: Light and Heat Feb. 15 A. B. C. D. E. x-rays ultraviolet light visible light radio waves None of the above--they all travel at the same speed. Feb. 15 3 Quiz QuesOon #2 Compared to light with a longer wavelength, light with a shorter wavelength is always A. B. C. D. redder. bluer. brighter. fainter. Key Concepts Light is electromagneOc radiaOon Hot things glow Color <--> temperature Why is the sky blue? Feb. 15 4 Feb. 15 5 Thought QuesOon The day side of the Moon has a temperature of about 107 C. The light we normally see from the Moon is reflected sunlight. If we want to see light emi^ed by the Moon, in what part of the electromagneOc spectrum should we observe? A. B. C. D. E. x-rays ultraviolet infrared radio I don't know how to answer this quesOon The speed of light is finite Olaus Rmer (1675) c = 3 108 m/s h^p://www.ux1.eiu.edu/~cfadd/1150/01Intro/std.html Feb. 15 6 Feb. 15 7 1 2/15/11 The spectrum of light Color purity Feb. 15 8 Feb. 15 9 Light acts like a wave ProperOes of waves Wikipedia frequency, (cycles per second, s-1, or Hz) Feb. 15 10 Feb. 15 = c (speed) 11 PercepOon Sound: amplitude = loudness frequency = pitch Light: amplitude = brightness frequency or wavelength = color shorter wavelength = bluer longer wavelength = redder Feb. 15 12 Feb. 15 13 2 2/15/11 What is "waving"? The electromagneOc spectrum h^p://www.nrc-cnrc.gc.ca/eng/projects/inms/opOcal-comb.html Feb. 15 14 Feb. 15 15 Heat is a form of energy Feb. 15 16 Feb. 15 17 Thought QuesOon Why does the interior of a car get so hot in the summer? Discussion QuesOon Why doesn't energy from the Sun make the Earth ho^er and ho^er? A. It does, but it takes millions of years for the temperature to increase by 1. B. As the Earth gets ho^er, clouds reflect more and more of the light. C. Ozone in the upper atmosphere absorbs all the Sun's heat. D. The Earth also loses heat by radiaOng in the infrared. E. All the heat is absorbed by the oceans. visible light, reflected infrared light, emi8ed 19 Feb. 15 18 Feb. 15 3 2/15/11 Hot things glow Discussion QuesOon Approximately what is the temperature of the glowing filament, in Kelvin? A. B. C. D. E. 5 K 50 K 500 K 5000 K 50000 K Feb. 15 20 Feb. 15 21 Temperature scales TF TC TK 9 TC + 32 5 5 = (TF - 32) 9 = TC + 273 = Wikipedia, "thermography" Feb. 15 22 Feb. 15 23 Blackbody spectrum Laws for blackbodies Wien's law relates the peak wavelength in the blackbody spectrum to temperature 0.0029 m K peak = T Stefan-Boltzmann law relates the energy flux* to temperature F = T4 Feb. 15 24 Feb. 15 = 5.67 10-8 W m-2 K-4
*Energy flux = Wa8s emi8ed by each square meter of the object's surface. 25 4 2/15/11 Example Suppose we double the (Kelvin) temperature of an object. What happens to its color and brightness? Color: Example What temperature is required to have a peak wavelength of 580 nm = 5.810-7 m? peak = 0.0029 m K T peak = Peak wavelength decreases by . Brightness: 0.0029 m K T 0.0029 m K T = peak 2.9 10-3 m K 5.8 10-7 m = 5000 K = F = T4 Brightness increases by 24 = 16. Feb. 15 26 Feb. 15 27 Spectrum of the Sun Discussion QuesOon Suppose two planets are the same size but different temperatures. Which of the following is true of the light emi^ed by the planets? A. B. C. D. E. The ho^er planet is fainter and bluer. The ho^er planet is fainter and redder. The ho^er planet is brighter and bluer. The ho^er planet is brighter and redder. The planets look the same. Feb. 15 28 Feb. 15 29 Why is the sky blue? Why is sunset red? Blackbody radiaOon from the Moon TC = 107 TK = TC + 273 = 380 0.0029 m K 380 K = 7.6 10-6 m = 7600 nm peak = Feb. 15 30 Feb. 15 33 5 ...
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