Physics 116 Session 30 Blackbody radiation and the photoelectric - - PowerPoint PPT Presentation

• R. J. Wilkes

Email: ph116@u.washington.edu

Physics 116

Session 30

Blackbody radiation and the photoelectric effect

Nov 18, 2011

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

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Lecture Schedule

(up to exam 3)

Today

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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:

– Gravitational time dilation and redshift – Deflection of light by gravity – Gravitational waves – Black holes

• Applications confirming GR today

– 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 *“If relativity is proved right, the Germans will call me a great German, the Swiss will call me a great Swiss, and the French will call me a great citizen of the world. If relativity is proved wrong, the French will call me a Swiss, the Swiss will call me a German, and the Germans will call me a Jew.” -Einstein

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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 * For more details on simultaneity, see http://en.wikipedia.org/wiki/Relativity_of_simultaneity

Spacetime diagram (in rest frame) of “simultaneous” events for two observers, 2nd has v=0.25c relative to 1st

v = 0.25c At rest Note: we assume earth and star are at rest relative to one another! Star’s worldline Event in rest frame occurs at different time in moving frame From each spacetime point in the rest frame we can draw a “line of simultaneity” for the moving frame

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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

• 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

“Red hot” glass

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“Blackbody” Spectrum (Planck Spectrum)

BB=object with 100% efficient emission and absorption at any wavelength Spectrum = graph of how much energy at each wavelength Blackbody spectrum features: Higher temperature means:

• 1. More total energy (=area

under curve): ETOT T4 2.Peak is at shorter wavelength (bluer color)

Kelvin temperature scale: uses Celsius- size degrees, but measures from absolute zero: 0 C = +273 K

physics.weber.edu/palen/Phsx1040/images/blackbody.jpg

Visible: 400—800 nanometer Examples: Surface of Sun = 6000K Carbon arc = 4000K Light bulb = 3000K IR UV

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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):
• Max Planck (Germany, 1901):

Found an empirical formula that approximates observations Found he could match

• bservations 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!

Closer look: Taking the temperature of the Universe

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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)

The cosmic microwave background (CMB)

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• 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
• rigins 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)

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First observed by Heinrich Hertz in 1887 - explained by Albert Einstein in 1905.

The Photoelectric Effect

Ammeter Battery

light Vacuum tube Flow of electrons = current Demonstration in class:

• Charge up an electroscope
• Bright light discharges it

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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!

13 Photons kick out electrons via photoelectric effect Photocathode: metallic salt coating on inside of glass tube Each energetic electron ejects about 4 new electrons at each “dynode” stage Vacuum inside tube Multiplied signal comes out here A + voltage between dynodes makes electrons accelerate from stage to stage

Photomultiplier Tube: application of photoelectric effect

• Crucial device for

medical imaging, basic research

• Can detect single

photons of UV light

• Photon’s arrival time

determined to nanosecond accuracy

• One photon in can be

multiplied to produce millions of electrons

• ut: easily measured

signals

High + voltage attracts and accelerates photoelectrons

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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

• 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

Quiz for today