Upcoming SlideShare
×

# 15 Mar 24 Interference, Diffraction

2,077 views

Published on

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

1 Like
Statistics
Notes
• Full Name
Comment goes here.

Are you sure you want to Yes No
• Be the first to comment

Views
Total views
2,077
On SlideShare
0
From Embeds
0
Number of Embeds
10
Actions
Shares
0
104
0
Likes
1
Embeds 0
No embeds

No notes for slide

### 15 Mar 24 Interference, Diffraction

1. 1. Today: Interferometers, Soap Bubbles, Small particle diffraction Glory from airplane. Colorful diffraction by Frank Peters on Flickr.
2. 2. Exam 2 results
3. 3. Clicker question—Single-Slit Diffraction <ul><li>Which of these series of pictures has the diffraction patterns correctly paired with the corresponding apertures? </li></ul>B A
4. 4. Clicker question—Single-Slit Diffraction <ul><li>Which of these series of pictures has the diffraction patterns correctly paired with the corresponding apertures? </li></ul>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
5. 5. Clicker question—Double-Slit Diffraction <ul><li>Which of these pictures shows double-slit diffraction? </li></ul>A B C http://www1.union.edu/newmanj/lasers /Light%20as%20a%20Wave/light_as_a_wave.htm
6. 6. Clicker question <ul><li>Which of these pictures shows double-slit diffraction? </li></ul>A B C Double-Slit (Two single-slit patterns Interfering with each other) Single-Slit Hexagonal Aperture
7. 7. 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
8. 8. Double-slit Interference <ul><li>Constructive interference occurs when the waves have traveled the same distance, or same distance plus one full wavelength. </li></ul><ul><li>Destructive interference occurs when path lengths traveled differ by ½ wavelength (so crest and valley super-impose) </li></ul>http://www.walter-fendt.de/ph14e/doubleslit.htm Ripple tank: http://www.youtube.com/watch?v=-8a61G8Hvi0&feature=related
9. 9. Clicker question—double slit interference <ul><li>If bright spots appear when path lengths are the same, is the spot centered between the two slits bright or dark? </li></ul><ul><li>bright </li></ul><ul><li>dark </li></ul>Bright or dark?
10. 10. Clicker question—double slit interference <ul><li>If bright spots appear when path lengths are the same, is the spot centered between the two slits bright or dark? </li></ul><ul><li>Bright (constructive) </li></ul><ul><li>dark </li></ul>Path lengths are equal
11. 11. Clicker question—double slit interference <ul><li>If bright spots appear when path lengths are the same, is the spot centered between the two slits bright or dark? </li></ul><ul><li>Bright (constructive) </li></ul><ul><li>dark </li></ul>Path lengths are equal Spacing depends on wavelength
12. 12. Double-slit interference can only be explained by waves. <ul><li>It happens even if one photon comes through at a time! (i.e. even for very dim light) </li></ul>
13. 13. <ul><ul><li>Since electrons behave like matter, I would expect to see two bands. </li></ul></ul><ul><ul><li>Since electrons behave like waves, I would expect to see an interference pattern </li></ul></ul>In the double slit experiment for electrons, do you expect to see two bands or do you expect an interference pattern?
14. 14. This little video explains the wave phenomena nicely <ul><li>This is an excellent clip from a goofy movie… (“what the bleep…”) </li></ul><ul><li>Beware of pseudo-science! </li></ul>
15. 15. Are electrons particles or waves? Hitachi double-slit with electrons http://www.hqrd.hitachi.co.jp/em/doubleslit.cfm
16. 16. Electrons (matter) also behave as waves! Hitachi double-slit with electrons http://www.hqrd.hitachi.co.jp/em/doubleslit.cfm
17. 17. Let’s do an experiment! <ul><li>Can we do single and double-slit diffraction with our microwave apparatus? </li></ul><ul><li>Laser – Can see light, but not easily apertures </li></ul><ul><li>Microwaves – Can see apertures, but not waves </li></ul><ul><li>Single slit diffraction Wavelength about 3 cm…how big should aperture be? </li></ul><ul><li>Double-slit diffraction how do we do it? </li></ul>
18. 18. Microwave Michelson Interferometer <ul><li>An interferometer is a device set up to measure the interference of two beams of light </li></ul>
19. 19. Clicker Question…Interferometer <ul><li>How will beams 1 and beams 2 interfere at the detector? </li></ul><ul><li>Constructively </li></ul><ul><li>Destructively </li></ul><ul><li>Not enough information </li></ul>
20. 20. Clicker Question…Interferometer <ul><li>Let’s make a hypothesis and try the experiment with microwaves! </li></ul>
21. 21. 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=RzZgFKoIfQI </li></ul>Also in space! LISA (2018)
22. 22. 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
23. 23. 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
24. 24. Information density is limited by laser spot size (and thus wavelength of laser) Blue lasers more difficult to make than red lasers
25. 25. Scattering off small particles also causes diffraction <ul><li>Remember the Brownian motion demo we did with wet paint? Laser speckle = interference </li></ul><ul><li>Comment on holography </li></ul><ul><li>Scattering from tiny water droplets or ice crystals causes all kinds of colorful effects in the sky http://www.atoptics.co.uk/ </li></ul>
26. 26. 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><ul><li>(Can we see this with stretched tube?) </li></ul>Fast Slow Fast
27. 27. Homework problem Thin film interference in your kitchen <ul><li>Requires black coffee mug, dish soap, sunlight </li></ul>