Like this presentation? Why not share!

# 15 Mar 24 Interference, Diffraction

## by Steve Koch, Professor at University of New Mexico on Apr 20, 2009

• 2,456 views

Single-slit diffraction, double-slit diffraction, thin film interference

Single-slit diffraction, double-slit diffraction, thin film interference

### Views

Total Views
2,456
Views on SlideShare
2,450
Embed Views
6

Likes
1
96
0

### Categories

Uploaded via SlideShare as Microsoft PowerPoint

## 15 Mar 24 Interference, DiffractionPresentation Transcript

• 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?
B A
• Clicker question—Single-Slit Diffraction
• Which of these series of pictures has the diffraction patterns correctly paired with the corresponding apertures?
B A 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 C http://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 patterns Interfering 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)
• 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?
• bright
• 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?
• Bright (constructive)
• 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?
• Bright (constructive)
• 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)
• Since electrons behave like matter, I would expect to see two bands.
• 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 electrons http://www.hqrd.hitachi.co.jp/em/doubleslit.cfm
• Electrons (matter) also behave as waves! Hitachi double-slit with electrons http://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 apertures
• Microwaves – Can see apertures, but not waves
• Single slit diffraction Wavelength about 3 cm…how big should aperture be?
• Double-slit diffraction how 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?
• Constructively
• Destructively
• 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
• Laser Gyroscopes
• Currently trying to detect gravitational waves
• LIGO
• LIGO video from 1:17 to 2:52
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 CD It’s a diffraction grating!
Diffraction Grating Tracks in a CD Spacing 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 sky http://www.atoptics.co.uk/
• Interference from thin films
• When 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 problem Thin film interference in your kitchen
• Requires black coffee mug, dish soap, sunlight