Lec10-Feb19 - 2/19/10 Discussion QuesJon Astro 109...

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Unformatted text preview: 2/19/10 Discussion QuesJon Astro 109 Lecture 10: Light and Heat Feb. 19 This star cluster contains many stars that look blue, plus one star that looks yellow. What can we infer about the temperatures of the stars? A.  B.  C.  D.  The yellow star is cooler than the blue stars. All the stars are the same temperature. The yellow star is hoHer than the blue stars. There is not enough informaJon to say anything about the temperatures. E.  I don’t know how to answer this quesJon. Feb. 19 Quiz QuesJon #1 Quiz QuesJon #2 The main reason you can see other people in this room now is because they Absolute zero temperature is A.  B.  C.  D.  E.  A.  B.  C.  D.  E.  emit electromagneJc radiaJon. absorb electromagneJc radiaJon. reflect electromagneJc radiaJon. scaHer electromagneJc radiaJon. cause the Doppler effect. Feb. 19 the coldest temperature ever achieved in a laboratory. the freezing point of water. the coldest outdoor temperature ever recorded in the U.S. the temperature at which atoms cease to move. 32° Fahrenheit. Feb. 19 Quiz QuesJon #3 Key Concepts •  Light is electromagneJc radiaJon Among glowing bodies with the following colors, which is the hoHest? A.  B.  C.  D.  E.  Feb. 19 red orange yellow blue black •  Hot things glow •  Color < ­> temperature Feb. 19 1 2/19/10 The speed of light is finite The spectrum of light c = 3 × 108 m/s Feb. 19 Feb. 19 Light acts like a wave Color purity Feb. 19 Feb. 19 Analogous behavior in water waves ProperJes of waves frequency, ν (cycles per second, s ­1, or Hz) Feb. 19 http://scripts.mit.edu/~tsg/www/demo.php?letnum=P%2011&show=1" Feb. 19 λ ν = c (speed) 2 2/19/10 Example WRSU ­FM broadcasts at 88.7 MHz. What is the wavelength of the radio waves? ν = 88.7 MHz = 88.7 × 106 s−1 = 8.87 × 107 s−1 c λ= ν 3 × 108 m/s = 8.87 × 107 s−1 = 3.4 m Feb. 19 PercepJon Sound: •  amplitude = loudness •  frequency = pitch Light: •  amplitude = brightness •  frequency or wavelength = color –  shorter wavelength = bluer –  longer wavelength = redder Feb. 19 ElectromagneJc waves What is “waving”? nanometer: 1 nm = 10−9 m Feb. 19 Feb. 19 Hot things glow Feb. 19 Feb. 19 3 2/19/10 Blackbody spectrum Feb. 19 Feb. 19 Temperature scales Discussion QuesJon Why does the heat from the Sun not make the Earth hoHer and hoHer? A.  It does, but it takes millions of years for the temperature to increase by 1°. B.  As the Earth gets hoHer, 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 radiaJng in the infrared. E.  All the heat is absorbed by the oceans. Feb. 19 TF TC TK 9 TC + 32 5 5 = (TF − 32) 9 = TC + 273 = Feb. 19 Example Laws for blackbodies •  Wien’s law relates the peak wavelength in the blackbody spectrum to temperature 0.0029 m K λpeak = T •  Stefan ­Boltzmann law relates the energy flux* to temperature F = σ T4 σ = 5.67 × 10−8 W m−2 K−4 Suppose we double the (Kelvin) temperature of an object. What happens to its color and brightness? Color: λpeak = 0.0029 m K T Peak wavelength decreases by ½. Brightness: F = σ T4 Brightness increases by 24 = 16. *Energy flux = Wa8s emi8ed by each Feb. 19 square meter of the object’s surface. Feb. 19 4 2/19/10 Discussion QuesJon Suppose two planets are the same size but different temperatures. Which of the following is true of the light emiHed by the planets? A.  B.  C.  D.  E.  The hoHer planet is fainter and bluer. The hoHer planet is fainter and redder. The hoHer planet is brighter and bluer. The hoHer planet is brighter and redder. The planets look the same. Feb. 19 Discussion QuesJon Suppose two stars are the same size and temperature, but star B is twice as far away as star A. Which of the following best describes how the stars will appear to us? A.  B.  C.  D.  Star B will appear bluer than star A. Star B will appear redder than star A. They will look idenJcal. They will have the same colors, but star B will appear fainter than star A. E.  They will have the same colors, but star B will appear brighter than star A. Feb. 19 ScaHering Spectrum of the Sun Feb. 19 Feb. 19 Example: HW #3 Approximately how many Jmes around the Earth could a beam of light travel in one second? diameter = 12576 km circumference = π d = 40000 km time Feb. 19 = 4 × 107 m distance = speed 4 × 107 m = 3 × 108 m s−1 = 0.13 s Feb. 19 5 2/19/10 Blackbody radiaJon from the human body Example: HW #7 A cellular phone is actually a radio transmiHer and receiver. You receive an incoming call in the form of a radio wave of frequency 880.65 MHz. What is the wavelength (in meters) of this wave? ν TF TC = 880.65 MHz TK = 880.65 × 106 s−1 = 8.8065 × 108 s−1 c λ= ν 3 × 108 m s−1 = 8.8075 × 108 s−1 = 0.34 m Feb. 19 λpeak = 98.6 5 = (TF − 32) = 37 9 = TC + 273 = 310 0.0029 m K 310 K = 9.35 × 10−6 m = = 9350 nm Feb. 19 6 ...
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This note was uploaded on 09/15/2011 for the course PHYS 109 taught by Professor Pryor during the Spring '09 term at Rutgers.

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