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Weekly Schedule Today • Finish LT and go over • Electromagne:c spectrum – Lecture Tutorials • Lab # 3-‐ Spectral Analysis Lab Next Thursday Hw # 3 Quiz # 3 Light cont. Doppler effect Reminder: Exam # 1 in one week! • I’ll post a study guide! • • • • • Electromagne:c radia:on Almost everything we know about the universe comes from analysis of EM I.e. The Transmission of energy through space Example: Light EM energy transmiQed as waves Need to know: wavelength, amplitude, period, frequency http://media.pearsoncmg.com/aw/aw_0media_astro/if/if.html?anatomy_of_a_wave 2.2 Waves in What? Water waves, sound waves, and so on, travel in a medium (water, air, …). Electromagnetic waves need no medium. Created by accelerating charged particles Rela:onships Frequency: Number of wave crests that pass a given point per second Period: Time between passage of successive crests Period = 1 / Frequency Wavelength: Distance between successive crests Velocity: Speed at which crests move Velocity = Wavelength / Period (Wavelength) x (Frequency) = velocity Frequency is measured in Hertz (cycles/second) Problem. • Find the wavelength of your groups favorite radio sta:on (measured in MHz) • Hint: Velocity (c)= 300,000 km/s f=c/λ Energy and wavelength are related…. E = h c λ 1 0.25 = 1 2 4 Inverse relationship 0.5 = E = energy h = Constant (Planck’s constant) c = constant (speed of light) λ = wavelength of photon (m) Cooler: -Longer wavelength (yellow) -Lower energy Hotter: -Shorter wavelength (blue) -Higher energy Energy and wavelength are related…. The Electromagnetic Spectrum Different colors of light are distinguished by their frequency and wavelength. The visible spectrum is only a small part of the total electromagnetic spectrum. Electromagne:c spectrum *Dis:nguished by frequency and wavelength http:// media.pearsoncmg.com/ aw/aw_0media_astro/if/ if.html?em_spectrum Things to note about the Eletromagnetic Spectrum • atmosphere is only transparent at a few wavelengths – the visible, the near infrared, and the part of the radio spectrum with frequencies higher than the AM band • atmosphere is absorbing a lot of the electromagnetic radiation impinging on it • Horizontal scale is logarithmic – each tick is a factor of 10 smaller or larger than the next one. http://media.pearsoncmg.com/aw/aw_0media_astro/if/if.html?light_absorption Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time. As the Sun gets lower, its light is passing through more of the atmosphere. Even more of the blue light is scattered, allowing the reds and yellows to pass straight through to your eyes. Which of the following travels the slowest? Radio waves Visible light X-‐rays Infrared light They all travel the same speed 57% 29% 14% T he y al e. .. sa m ve l t he l t ra In f ra re d li gh t ys 0% X -‐ra ht lig e bl V isi ad io w av es 0% R 1. 2. 3. 4. 5. Which of the following is not a form of light? Radio waves Microwaves X-‐rays All of the above None of the above 57% 36% 7% bo ve e f t h e a bo v N on e o he a of t X -‐ra ys 0% A ll icr M ad io w ow av av e s es 0% R 1. 2. 3. 4. 5. Radia:on Laws • Temp-‐ amount of mo:on • F = 9/5C + 32 • Kelvins = C + 273 Thermal Radiation Intensity: amount or strength of radiation Blackbody spectrum: Radiation emitted by an object over a range of frequencies * Not symmetrical; falloff more rapid at high frequency side for all object Blackbody curves Energy output (y-‐axis) Wavelength (x-‐axis) *Can reveal temperature (and size) informa:on *No:ce how the curves shii towards higher frequencies with increasing temperature. What’s the wavelength doing? http://media.pearsoncmg.com/aw/ aw_0media_astro/if/if.html?wiens_law Wien’s Radia:on Laws: Peak wavelength α 1/temperature Stefan’s Law: Energy α temperature4 • The star Veritas is observed to have a blue-‐white light of peak wavelength 450 nm, and the star Erasmus is observed to have a deep reddish light of peak wavelength 720 nm. • Which star is hoQer? HoQer it is, the bluer it is, the more energy it has The graph at right shows the blackbody spectra for three different stars. Which of the stars is at the highest temperature? 58% Peak wavelength ~ 1/temperature Energy ~ temperature4 33% ar C St ar B St ar A 8% St 1. Star A 2. Star B 3. Star C A lump of lead is heated to a high temperature. Another lump of lead that is twice as large is heated to a lower temperature. Which lump of material appears bluer? 100% The cooler lump appears bluer. The hoQer lump appears bluer. Both lumps look the same color. Cannot tell which lump looks bluer. p .. m sa lu th e C an no t t e ll w hi ch ok ps lo 0% . 0% .. s.. . pe ar p ap ot h lu m B te r l um ho t he T he co ol er lu m p a pp e ar s.. . 0% T 1. 2. 3. 4. Astronomical Implica:ons • Young stars emit infrared/radio radia:on because they are cooler • The brightest stars emit ultraviolet radia:on because of their high surface temperature http://media.pearsoncmg.com/aw/aw_chaisson_astronomytoday_6/videos/ch03/ OrionNebMultispectral.html PRODUCING A CONTINUOUS SPECTRUMSpectroscope: Splits light into component colors A continuous spectrum is produced by a hot, high-density light source. PRODUCING AN EMISSION (Light) SPECTRUM Emission lines: Single frequencies emitted by particular atoms What’s the black showing? All wavelengths not emitted! An emission spectrum is produced by a hot, low-density light source. EMISSION SPECTRA TELL US CHEMICAL COMPOSITION OF STARS Figure 2.16 PRODUCING AN ABSORPTION (Dark line) SPECTRUM An absorption spectrum is produced by a hot, high-density light source shining through a cool, low-density medium. Or by looking at the SUN through a prism!!! http://media.pearsoncmg.com/aw/ aw_0media_astro/if/if.html? illustrating_kirchhofs_laws Absorption spectrum: If a continuous spectrum passes through a cool gas, atoms of the gas will absorb the same frequencies they emit. THIS IS THE SUN’S SPECTRUM. THE SUN’S SPECTRUM IS ______. Which of the following spectra is produced by the Sun? 90% 1. Dark line absorp:on spectrum 2. Bright line emission spectrum 3. Con:nuous spectrum 10% m tru ec us sp C on t in uo em iss io e ht lin rig B D ar k lin e ab so rp t io n sp ... n sp e. .. 0% e. .. uo us s nt in co b r ig ht lin e e m is s io pe ct n sp n sp .. . so rp tio ab ne k l i d ar ru m Imagine that you observe the Sun while in your space ship far above Earth’s atmosphere. Which of the following spectra would you observe by analyzing the sunlight? 50% 1. dark line absorp:on spectrum 33% 2. bright line emission spectrum 3. con:nuous spectrum 17% Imagine that you are on the surface of Earth (below the atmosphere) and are observing the 89% Sun. Which of the following spectra would you observe by analyzing the sunlight? 1. dark line absorp:on spectrum 2. bright line emission spectrum 3. con:nuous spectrum uo us s nt in co b r ig ht lin e e m is s io pe ct n sp n sp .. . e. .. ru m 0% so rp tio ab ne k l i d ar 11% Stop Bohr’s Model of the Atom explained the observations of “Line Spectra” ~700nm A M F M T V Micr owav e Infrared Ultraviolet ~350nm X-Rays Gamma-Rays Bohr’s model of the atom This model of the atom nicely explains many phenomena. For example: Emission Spectra As you heat up an atom, all the electrons absorb the heat and thus gain Kinetic Energy. This moves the electrons out to higher orbitals. As you heat up an atom, all the electrons absorb the heat and thus gain Kinetic Energy. This moves the electrons out to higher orbitals. As electrons are allowed only in certain orbitals, they can only fall certain, well defined “distances”. This gives the us the well defined spectral lines that we saw in lab yesterday. Bohr’s Model of the Atom explained the observations of “Line Spectra” ~700nm A M F M T V Micr owav e Infrared Ultraviolet ~350nm X-Rays Gamma-Rays Which drawing represents the process by which and absorp:on line is formed? 1. 2. 3. 4. A B C D Which drawing represents the process by which an emission line is formed? 1. 2. 3. 4. A B C D 10 Countdown Is this atom absorbing or emitting energy? What type of spectrum will it produce? Doppler Effect Red shii = moving away Blue shii = moving towards Measured in a lab A bright star is moving away from Earth. Which of the choices best completes the following statement describing the spectrum of this star? A(n) ___________ spectrum that is _______ rela:ve to 20% 20% 20% 20% 20% an unmoving star. pt io n; re co ... nt in uo us ; r e. .. bl . .. bs or uo us ; a co nt in n; re ds . .. iss io e m bs or pt io n; b l.. . absorp:on; blueshiied emission; redshiied con:nuous; blueshiied absorp:on; redshiied con:nuous; redshiied a 1. 2. 3. 4. 5. 10 Countdown Which object is at rest? T n. .. ov i re m he y a O bj ec t D O bj ec t C 20% 20% 20% 20% 20% O bj ec t B Object A Object B Object C Object D They are moving the same speed, the speed of light O bj ec t A 1. 2. 3. 4. 5. 10 Countdown Which of the objects is moving with the fastest speed? n. .. ov i re m 10 T he y a O bj ec t D O bj ec t C 20% 20% 20% 20% 20% O bj ec t B Object A Object B Object C Object D They are moving the same speed, the speed of light O bj ec t A 1. 2. 3. 4. 5. Countdown