New Reading, Exercises, HW Activity, and Quiz posted this weekend

Today: Soap Bubble, Photoelectric Effect, Electron Waves

Interferometers are great physics tools

Proved there is no “luminiferous ether”Michelson-Morley Experiment 1887

http://www.youtube.com/watch?v=7ORSeqytMR8&NR=1

Laser Gyroscopes

Currently trying to detect gravitational wavesLIGOLIGO video from 1:17 to 2:52http://www.youtube.com/watch?v=UxpMTj2pHO4

Also in space!LISA (2018)

Much of the physics of polarized light, diffraction and interference are illustrated by a CD & DVD Player!

Look at light diffracted off a CDIt’s a diffraction grating!

Diffraction Grating

Tracks in a CDSpacing about ½ micron

Pits in a CD

Much of the physics of polarized light, diffraction and interference are illustrated by a CD & CD Player!

A CD Player uses polarized laser light for reading the CD

http://hyperphysics.phy-astr.gsu.edu/HBASE/audio/cdplay.html

Destructive interference for detection

Information density is limited by laser spot size (and thus wavelength of laser)

Blue lasers more difficult to make than red lasers

Interference from thin filmsWhen a wave reflects from a boundary between

fast and slow, there is a 180 degree phase shift

Fast

Slow

Fast

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 throughinterfaces.

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

180 degree phase shift explains the black color in the thinnest part of the soap film

Thinnest part of 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

Clicker Question—Thin film interference

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?

A) Destructive B) Constructive

GLASS (Slower than water)

Clicker Question—Thin film interference

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?

A) Destructive B) ConstructiveBoth the top and bottom reflection would undergo a 180 degree phase

shift

GLASS (Slower than water)

Let’s look at a soap film demo

Fast

Slow

Fast

Let’s Brainstorm: Everyday examples of interference?Where in nature do you see sunlight (or indoor lights) being spread into colors?

Sprinklers, rainbows, waterfalls, diffraction grating

Bubbles, oil caked onto a cooking pan (thin film of oil) (gasoline on pavement), prisms ,

So, we have seen a bunch of demonstrations of wave properties of light…

Now we will see that light behaves like a particle too!

Einstein’s 1905, It was a very good year!

March, 1905 – Photoelectric effect explained by “light quanta” (photons)

May 1905 – Mathematical explanation of Brownian motion

June 1905 – Relativity principle (special relativity)…new understanding of space and time.

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

– Interference patterns (soap bubbles, etc)– Reflection and refraction– Huygens' principle– Diffraction

What evidence do we have that light acts like a wave?

– Key point: The energy of a wave can be increased by increasing the amplitude of the wave.

– Any frequency wave can have any amount of energy!

– Max Planck suggested this in 1901.– Einstein worked out mathematics in 1905

– Each packet would carry an energy, E = h f– h is Planck's constant

What if light were made of individual packets of energy, called quanta?

Key point: The energy of a single packet (photon) can only be increased if the frequency is increased.

– It is possible to produce free electrons by shining light on a sheet of metal.

– The electrons need to receive a certain threshold amount of energy to be freed from the metal and travel through the air.

How can we tell if Planck was right?

– What results would we expect if light behaves like a wave?

– What about if it behaves like a packet of energy?

– What results would we expect if light behaves like a wave?

– What about if it behaves like a packet of energy?

– 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.

– If light behaves like particles, then only frequencies higher than a certain threshold should eject electrons and produce current.

– The light will behave like a wave: it will be possible to eject electrons at all frequencies.

– The light will behave like particles: it will be possible to eject electrons only at frequencies higher than a threshold.

Clicker Question: How do you think light will behave when we shine it on a

metal?

Let’s try our own demo of the photoelectric effect

First, let’s try the experiment with an applet to get familiar with apparatushttp://www.ifae.es/xec/phot2.html

Explanatory video: http://www.youtube.com/watch?v=N7BywkIretM

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

The photoelectric effects proves that light comes in discrete packets (photons)

Let’s think about waves first.

Imagine this fishing bobber.What do we need to change

about the wave to make thebobber go high enough to gothrough the ring?

The photoelectric effects proves that light comes in discrete packets (photons)

Let’s think about waves first.

Now, instead thing about lightshining on fluorescent molecules

Can bright light of any frequencyexcite the electron? NO

Clicker question—photoelectric effect

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?

A) Red light will eject approximately ½ as many electrons as blue light

B) Blue light will eject way, way more electrons than red light

C) They will eject almost the same amount of electrons!

MetalAir

Ene

rgy

Red photon energyBlue photon energy

Photon energy = h * frequency

Clicker question—photoelectric effect

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?

A) Red light will eject approximately ½ as many electrons as blue light

B) Blue light will eject way, way more electrons than red light

C) They will eject almost the same amount of electrons!

MetalAir

Ene

rgy

Red photon energyBlue photon energy

Photon energy = h * frequency

Take-home message from all of this:

We can only understand the physics of light if it is both a wave and a particle. (Sorry!)

Thinnest part of film

Photoelectric effectDiffraction

Interference

Fluorescence

Clicker Question--Electrons

Are electrons particles or waves?

A) Particles—duh!B) Waves?C) Both???D) NeitherE) Yes

Clicker Question--Electrons

Are electrons particles or waves?

A) Particles—duh!B) Waves?C) Both???D) NeitherE) Yes

It turns out that matter (electrons) behaves like waves and particles too!

Well, if electrons can behave like waves…

Shouldn’t we be able to refract / focus the electrons?

Yes…”Electron Microscopy!”

http://www.columbia.edu/~ac558/jj1.jpg

Electron microscopy particularly useful for surface science

Single-atom thick silicon islandsGayle Thayer—Sandia / IBM

IBM—Pentacene islands on Silicon(Imaging photoelectric effect!, PEEM)

Boron growth on silicon--Sandia

Next up is nuclear physics!!

Homework problemThin film interference in your kitchenMake your own videos / pictures!

Requires black coffee mug, dish soap, sunlight