Today: Interferometers, Soap Bubbles, Small particle diffraction

Glory from airplane. Colorful diffraction by Frank Peters on Flickr.

Exam 2 results

Clicker question—Single-Slit Diffraction

Which of these series of pictures has the diffraction patterns correctly paired with the corresponding apertures?

BA

Clicker question—Single-Slit Diffraction

Which of these series of pictures has the diffraction patterns correctly paired with the corresponding apertures?

BA

Smaller aperture = MORE like a point source. Therefore, MORE spreading.

Aperture much wider than original beam -> should not affect the beam

http://www.lon-capa.org/~mmp/kap27/Gary-Diffraction/app.htm

Clicker question—Double-Slit Diffraction

Which of these pictures shows double-slit diffraction?

A B

Chttp://www1.union.edu/newmanj/lasers/Light%20as%20a%20Wave/light_as_a_wave.htm

Clicker question

Which of these pictures shows double-slit diffraction?

A B

C

Double-Slit(Two single-slit patternsInterfering with each other)

Single-Slit

Hexagonal Aperture

Because of interference, the double slite diffraction pattern is not simply the “sum” of two single-slits

Single-Slit #1+

Single-Slit #2

Double-Slit

http://www.saburchill.com/physics/chapters2/00081.html

Double-slit Interference

Constructive interference occurs when the waves have traveled the same distance, or same distance plus one full wavelength.

Destructive interference occurs when path lengths traveled differ by ½ wavelength (so crest and valley super-impose)

http://www.walter-fendt.de/ph14e/doubleslit.htm

Ripple tank: http://www.youtube.com/watch?v=-8a61G8Hvi0&feature=related

Clicker question—double slit interference

If bright spots appear when path lengths are the same, is the spot centered between the two slits bright or dark?

A. brightB. dark

Bright or dark?

Clicker question—double slit interference

If bright spots appear when path lengths are the same, is the spot centered between the two slits bright or dark?

A. Bright (constructive)B. dark

Path lengths are equal

Clicker question—double slit interference

If bright spots appear when path lengths are the same, is the spot centered between the two slits bright or dark?

A. Bright (constructive)B. dark

Path lengths are equal

Spacing depends on wavelength

Double-slit interference can only be explained by waves.

It happens even if one photon comes through at a time! (i.e. even for very dim light)

A) Since electrons behave like matter, I would expect to see two bands.

B) Since electrons behave like waves, I would expect to see an interference pattern

In the double slit experiment for electrons, do you expect to see two bands or

do you expect an interference pattern?

This little video explains the wave phenomena nicely

This is an excellent clip from a goofy movie… (“what the bleep…”)

Beware of pseudo-science!

Are electrons particles or waves?

Hitachi double-slit with electronshttp://www.hqrd.hitachi.co.jp/em/doubleslit.cfm

Electrons (matter) also behave as waves!

Hitachi double-slit with electronshttp://www.hqrd.hitachi.co.jp/em/doubleslit.cfm

Let’s do an experiment!

Can we do single and double-slit diffraction with our microwave apparatus?Laser – Can see light, but not easily aperturesMicrowaves – Can see apertures, but not waves

Single slit diffractionWavelength about 3 cm…how big should aperture be?

Double-slit diffractionhow do we do it?

Microwave Michelson Interferometer

An interferometer is a device set up to measure the interference of two beams of light

Clicker Question…Interferometer

How will beams 1 and beams 2 interfere at the detector?

A. ConstructivelyB. DestructivelyC. Not enough information

Clicker Question…Interferometer

Let’s make a hypothesis and try the experiment with microwaves!

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=RzZgFKoIfQI

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

Scattering off small particles also causes diffraction

Remember the Brownian motion demo we did with wet paint?Laser speckle = interference

Comment on holography

Scattering from tiny water droplets or ice crystals causes all kinds of colorful effects in the skyhttp://www.atoptics.co.uk/

Interference from thin filmsWhen a wave reflects from a boundary between

fast and slow, there is a 180 degree phase shift(Can we see this with stretched tube?)

Fast

Slow

Fast

Homework problemThin film interference in your kitchen

Requires black coffee mug, dish soap, sunlight