15 Mar 24 Interference, Diffraction
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Transcript of 15 Mar 24 Interference, Diffraction
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