Physics 116 Session 30 Blackbody radiation and the photoelectric effect Nov 18, 2011 R. J. Wilkes Email: ph116@u.washington.edu
Announcements: • � Updated quiz score totals will be posted on WebAssign tomorrow • � Nice series on PBS covering topics we will discuss in class: Brian Greene’s Fabric of the Cosmos http://www.pbs.org/wgbh/nova/physics/fabric-of-cosmos.html
Lecture Schedule (up to exam 3) Today 3
General relativity • � Einstein, 1915: extended relativity to accelerated frames: general relativity – � GR really describes the geometry of spacetime: gravity of massive objects warps spacetime in their vicinity – � Equivalence Principle: Observations cannot distinguish a uniformly accelerated frame from a uniform gravity field – � Eddington, 1919: GR predictions matched observed anomalies in orbit of Mercury, Newtonian predictions do not – Einstein is right* • � More predictions and consequences of GR: *“If relativity is proved right, the Germans will – � Gravitational time dilation and redshift call me a great German, the Swiss will call me a – � Deflection of light by gravity great Swiss, and the French will call me a great citizen of the world. – � Gravitational waves If relativity is proved wrong, the French will call – � Black holes me a Swiss, the Swiss will call me a German, • � Applications confirming GR today and the Germans will call me a Jew.” -Einstein – � GPS satellite orbits: precision needed requires GR calculations – � Gravitational lensing, black holes: astronomical observations confirm – � Gravitational wave astronomy: see http://www.ligo-la.caltech.edu/LLO/overviewsci.htm – � Notice: LIGO is a variety of Michelson apparatus! • � We’re still looking for unexplained anomalies: UW is a center for this work – � See http://www.npl.washington.edu/eotwash/index.html 4
Worldlines for the twin paradox • � Diagram shows worldlines for the twins – � Notice travelling twin has to jump from going to coming = acceleration! – � This means travelling twin observes an apparent jump in age of his twin between arrival and departure from star • � In relativity there is no absolute “right now” – � Events that are simultaneous in one frame may occur at different times in another* – � Lines of simultaneity are tilted according to relative speed Spacetime diagram (in rest frame) of “simultaneous” events for two observers, 2 nd has v=0.25c relative to 1 st Event in rest frame occurs at different time in moving frame v = 0.25c At rest Star’s Note: we assume earth worldline and star are at rest From each spacetime point in the rest frame we can relative to one another! draw a “line of simultaneity” for the moving frame * For more details on simultaneity, see http://en.wikipedia.org/wiki/Relativity_of_simultaneity 5
Blackbody radiation: another 1890s puzzle • � Any object’s molecules are vibrating in place …As long as its temperature is above “absolute zero” : 0° K = - 273° C “Red hot” glass • � Atoms are made of charged particles – � So they emit E-M radiation – � Frequency of emission depends on motion – � Total radiation from any object covers a broad range of frequencies (wavelengths): random mix of molecular speeds • � Calculated spectrum (graph of intensity vs wavelength) from an ideal radiator is called “blackbody spectrum” – � Ideal radiator = ideal radiation absorber – � Color of an object = color of light it reflects (does not absorb) • � So, what color would an ideal absorber appear to be? Experimental approximation for a blackbody Metal cavity with pinhole: any light that enters is unlikely to escape before walls absorb its energy ~ perfect absorber 6
“Blackbody” Spectrum (Planck Spectrum) BB=object with 100% efficient emission and absorption at any wavelength Kelvin temperature UV IR scale: uses Celsius- size degrees, but Examples: measures from Surface of Sun = 6000K absolute zero: Carbon arc = 4000K 0 C = +273 K Light bulb = 3000K Spectrum = graph of how much energy at each wavelength Blackbody spectrum features: Higher temperature means: 1. � More total energy (=area under curve): E TOT � T 4 2. � Peak is at shorter wavelength (bluer color) Visible: 400—800 nanometer physics.weber.edu/palen/Phsx1040/images/blackbody.jpg 7
Classical prediction doesn’t match observations • � According to Maxwell’s equations and 1890s thermodynamics, emission intensity should rise rapidly with frequency – any real BB would have to emit infinite total energy (= area under spectrum curve) • � “Ultraviolet catastrophe”! • � W. Wien (1896): Found an empirical formula that approximates observations • � Max Planck (Germany, 1901): Found he could match observations precisely if he made a simple assumption: suppose atoms can emit energy only in units (“quanta”) with size depending on frequency: Planck’s constant: very tiny on human scale! 8
Closer look: Taking the temperature of the Universe Spectrum of radiation from “empty space” matches the Planck BB spectrum for 2.725 deg K very precisely (error bars are tiny compared to dots: about 0.05 units) 9
The cosmic microwave background (CMB) • � Sky map of the Cosmic Microwave Background (from NASA’s WMAP satellite - “equator” = plane of our Galaxy) Each dot = measured temperature in that direction on the sky • � Color range from red to dark blue = a variation of only + 0.0002 degrees K from 2.75K average temperature! • � Even these tiny variations are meaningful: fluctuations represent origins of galaxies! Sky map: same idea as map of the world, but looking up at stars, not down on Earth. Here, “equator” = Galactic Plane (our galaxy = Milky way) 10
The Photoelectric Effect First observed by Heinrich Hertz in 1887 - explained by Albert Einstein in 1905. light Vacuum tube Ammeter Battery Flow of electrons = current Demonstration in class: • � Charge up an electroscope • � Bright light discharges it 11
Einstein’s explanation • � Let’s take Planck’s quantum idea seriously! (sound familiar?) – � Not just a math trick that fixes up the blackbody spectrum puzzle • � Suppose energy in the form of light really does come in quanta? – � Planck said: violet light quanta have more E than red quanta Planck’s law: E = h f = h c/ ! � red light = long wavelengths, violet = short – � Quantum concept means energy is delivered in bundles, not continuously, as with waves – � Electrons cannot ‘soak up’ energy: each photon (Einstein’s new term for quanta or ‘particles’ of light) transfers a lump of energy all at once – � Only short ! photons carry enough energy per quantum to knock an electron loose (negative charges had been identified as electrons by Thompson) – � Long ! photons can never kick electrons loose: too little E/photon – � Intense light means many photons but not more energy per photon! 12
Photomultiplier Tube: application of photoelectric effect Photons kick out • � Crucial device for electrons via Photocathode: metallic salt photoelectric effect medical imaging, coating on inside of glass tube basic research • � Can detect single High + voltage photons of UV light attracts and • � Photon’s arrival time accelerates photoelectrons determined to nanosecond accuracy Each energetic • � One photon in can be electron ejects about 4 new multiplied to produce electrons at each Vacuum inside tube millions of electrons “dynode” stage out: easily measured signals A + voltage between Multiplied signal dynodes makes comes out here electrons accelerate from stage to stage 13
Photomultipliers = everyday technology • � Every time we use one, we prove Einstein was right… 1” diameter PMT Array of hexagonal 2” PMTs used in medical imaging 14
Quiz for today • � If I stand still on Earth, and you go past me in a spaceship moving with v = 0.99c A. � I say your clock runs slow relative to mine B. � You say my clock runs slow relative to yours C. � Both of the above are true D. � Neither A nor B are true
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