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Today: Soap Bubble, Photoelectric Effect, Electron Waves New Reading, Exercises, HW Activity, and Quiz posted this weekend

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Interferometers are great physics tools <ul><li>Proved there is no “luminiferous ether” </li></ul><ul><li>Michelson-Morley Experiment 1887 </li></ul><ul><li>http://www.youtube.com/watch?v=7ORSeqytMR8&NR=1 </li></ul><ul><li>Laser Gyroscopes </li></ul><ul><li>Currently trying to detect gravitational waves </li></ul><ul><li>LIGO </li></ul><ul><li>LIGO video from 1:17 to 2:52 </li></ul><ul><li>http://www.youtube.com/watch?v=UxpMTj2pHO4 </li></ul>Also in space! LISA (2018)

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Much of the physics of polarized light, diffraction and interference are illustrated by a CD & DVD Player! <ul><li>Look at light diffracted off a CD It’s a diffraction grating! </li></ul>Diffraction Grating Tracks in a CD Spacing about ½ micron Pits in a CD

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Much of the physics of polarized light, diffraction and interference are illustrated by a CD & CD Player! <ul><li>A CD Player uses polarized laser light for reading the CD </li></ul>http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/cdplay.html Destructive interference for detection

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Information density is limited by laser spot size (and thus wavelength of laser) Blue lasers more difficult to make than red lasers

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Interference from thin films <ul><li>When a wave reflects from a boundary between fast and slow, there is a 180 degree phase shift </li></ul>Fast Slow Fast

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A soap bubble or film is a thin layer of water stabilized by detergent molecules http://www.funsci.com/fun3_en/exper2/exper2.htm Water: speed of light slower Air: speed of light fast Air Incident light will reflect and pass through interfaces. Reflections off the top surface and bottom surface will interfere with each other KEY POINT: 180 degree phase shift when reflecting off interface going from fast to slow (top interface in this case) Detergent layer

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180 degree phase shift explains the black color in the thinnest part of the soap film http://www.funsci.com/fun3_en/exper2/exper2.htm Water: speed of light slower Air: speed of light fast Air When film is very thin, path length difference is negligible. So interference is destructive . Appears black because I used black mug. As film gets thicker, there is some path length difference, so interference is not completely destructive – white color. Wikipedia.org Detergent layer

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Clicker Question—Thin film interference <ul><li>What if we had a thin layer of water on glass? In a very thin film (say 20 nanometers), would there be destructive or constructive interference? </li></ul><ul><li>A) Destructive B) Constructive </li></ul>GLASS (Slower than water)

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Clicker Question—Thin film interference <ul><li>What if we had a thin layer of water on glass? In a very thin film (say 20 nanometers), would there be destructive or constructive interference? </li></ul><ul><li>A) Destructive B) Constructive </li></ul><ul><li>Both the top and bottom reflection would undergo a 180 degree phase shift </li></ul>GLASS (Slower than water)

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<ul><li>Let’s look at a soap film demo </li></ul>Fast Slow Fast

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Let’s Brainstorm: Everyday examples of interference? Where in nature do you see sunlight (or indoor lights) being spread into colors? <ul><li>Sprinklers, rainbows, waterfalls, diffraction grating </li></ul><ul><li>Bubbles, oil caked onto a cooking pan (thin film of oil) (gasoline on pavement), prisms , </li></ul>

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So, we have seen a bunch of demonstrations of wave properties of light… <ul><li>Now we will see that light behaves like a particle too! </li></ul>

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Einstein’s 1905, It was a very good year! <ul><li>March, 1905 – Photoelectric effect explained by “light quanta” (photons) </li></ul><ul><li>May 1905 – Mathematical explanation of Brownian motion </li></ul><ul><li>June 1905 – Relativity principle (special relativity)…new understanding of space and time. </li></ul>

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What evidence do we have that light acts like a wave? Brainstorm! (Thanks to Katie Richardson-McDaniel for next many slides)

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<ul><ul><li>Interference patterns (soap bubbles, etc)‏ </li></ul></ul><ul><ul><li>Reflection and refraction </li></ul></ul><ul><ul><li>Huygens' principle </li></ul></ul><ul><ul><li>Diffraction </li></ul></ul>What evidence do we have that light acts like a wave? <ul><ul><li>Key point: The energy of a wave can be increased by increasing the amplitude of the wave. </li></ul></ul><ul><ul><li>Any frequency wave can have any amount of energy! </li></ul></ul>

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<ul><ul><li>Max Planck suggested this in 1901. </li></ul></ul><ul><ul><ul><li>Einstein worked out mathematics in 1905 </li></ul></ul></ul><ul><ul><li>Each packet would carry an energy, E = h f </li></ul></ul><ul><ul><li>h is Planck's constant </li></ul></ul>What if light were made of individual packets of energy, called quanta? <ul><ul><li>Key point: The energy of a single packet (photon) can only be increased if the frequency is increased. </li></ul></ul>

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<ul><ul><li>It is possible to produce free electrons by shining light on a sheet of metal. </li></ul></ul><ul><ul><li>The electrons need to receive a certain threshold amount of energy to be freed from the metal and travel through the air. </li></ul></ul>How can we tell if Planck was right? <ul><ul><li>What results would we expect if light behaves like a wave? </li></ul></ul><ul><ul><li>What about if it behaves like a packet of energy? </li></ul></ul>

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<ul><ul><li>What results would we expect if light behaves like a wave? </li></ul></ul><ul><ul><li>What about if it behaves like a packet of energy? </li></ul></ul><ul><ul><li>If light behaves like a wave , then any frequency of light should be able to eject electrons if we dial up the amplitude of the wave (and therefore the intensity) high enough. </li></ul></ul><ul><ul><li>If light behaves like particles , then only frequencies higher than a certain threshold should eject electrons and produce current. </li></ul></ul>

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<ul><ul><li>The light will behave like a wave : it will be possible to eject electrons at all frequencies . </li></ul></ul><ul><ul><li>The light will behave like particles : it will be possible to eject electrons only at frequencies higher than a threshold . </li></ul></ul>Clicker Question: How do you think light will behave when we shine it on a metal?

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Let’s try our own demo of the photoelectric effect First, let’s try the experiment with an applet to get familiar with apparatus http://www.ifae.es/xec/phot2.html Explanatory video: http://www.youtube.com/watch?v=N7BywkIretM

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Post-Experiment Discussion Point: Does this finding change the wave-like observations we've made before?

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The photoelectric effects proves that light comes in discrete packets (photons) <ul><li>Let’s think about waves first. </li></ul><ul><li>Imagine this fishing bobber. </li></ul><ul><li>What do we need to change about the wave to make the bobber go high enough to go through the ring? </li></ul>

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The photoelectric effects proves that light comes in discrete packets (photons) <ul><li>Let’s think about waves first. </li></ul><ul><li>Now, instead thing about light shining on fluorescent molecules </li></ul><ul><li>Can bright light of any frequency excite the electron? NO </li></ul>

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Clicker question—photoelectric effect <ul><li>Light is made up of particles (photons). The electrons can only absorb one whole photon at a time. If we have red and blue light of equal intensity, which of the following is true? </li></ul><ul><li>Red light will eject approximately ½ as many electrons as blue light </li></ul><ul><li>Blue light will eject way, way more electrons than red light </li></ul><ul><li>They will eject almost the same amount of electrons! </li></ul>Red photon energy Blue photon energy Photon energy = h * frequency Metal Air Energy

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Clicker question—photoelectric effect <ul><li>Light is made up of particles (photons). The electrons can only absorb one whole photon at a time. If we have red and blue light of equal intensity, which of the following is true? </li></ul><ul><li>Red light will eject approximately ½ as many electrons as blue light </li></ul><ul><li>Blue light will eject way, way more electrons than red light </li></ul><ul><li>They will eject almost the same amount of electrons! </li></ul>Red photon energy Blue photon energy Photon energy = h * frequency Metal Air Energy

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Take-home message from all of this: <ul><li>We can only understand the physics of light if it is both a wave and a particle . (Sorry!) </li></ul>Photoelectric effect Diffraction Interference Fluorescence

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Clicker Question--Electrons <ul><li>Are electrons particles or waves? </li></ul><ul><li>Particles—duh! </li></ul><ul><li>Waves? </li></ul><ul><li>Both??? </li></ul><ul><li>Neither </li></ul><ul><li>Yes </li></ul>

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Clicker Question--Electrons <ul><li>Are electrons particles or waves? </li></ul><ul><li>Particles—duh! </li></ul><ul><li>Waves? </li></ul><ul><li>Both??? </li></ul><ul><li>Neither </li></ul><ul><li>Yes </li></ul>It turns out that matter (electrons) behaves like waves and particles too!

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Well, if electrons can behave like waves… <ul><li>Shouldn’t we be able to refract / focus the electrons? </li></ul><ul><li>Yes… ”Electron Microscopy!” </li></ul>http://www.columbia.edu/~ac558/jj1.jpg

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Electron microscopy particularly useful for surface science Single-atom thick silicon islands Gayle Thayer—Sandia / IBM IBM—Pentacene islands on Silicon (Imaging photoelectric effect!, PEEM) Boron growth on silicon--Sandia

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Next up is nuclear physics!! Homework problem Thin film interference in your kitchen Make your own videos / pictures! <ul><li>Requires black coffee mug, dish soap, sunlight </li></ul>