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24

VIEWS: 3 PAGES: 19

									              24 Overview
• wave effects:
• interference, diffraction
• polarization, scattering

• Homework:
• 1, 12, 43, 57, 67.


                              1
                waves
• pass through another wave undisturbed
• diffract video demo simulation
• Reflect wave tank demo




                                          2
             Interference
• amplitude change due to adding waves
• correlation (Coherence) - required for
  strong interference effects
• misalignment of waves (phase) is zero for
  constructive (bright)
• 180 deg: destructive (dark)
• Wave tank demo

                                              3
       double-slit experiment
• coherent light
• Interference pattern
• path difference




                                4
naming convention


            m = 2 bright fringe
           m = 1 bright fringe
  L
           central max
           m = 1 dark fringe
           m = 2 dark fringe
           m = 3 dark fringe



                                  5
  Double-Slit Phase Difference
• Bright: (dsinq) = 0, 1, 2, … wavelengths
• Dark: (dsinq)= ½, 1½, 2½, … wavelengths


 Bright     d sin q  m        m  1, 2, 3,...

 Dark     d sin q  (m  1 )
                         2       m  1, 2, 3,...

                                                   6
Single Slit Pattern




                      7
8
               Polarizing Filters
• Polarizing material
  allows the passage
  of only one direction
  of E
• Malus’ Law:

       I  I 0 cos 2 q




                                    9
      Two Filters (incident light
           unpolarized)
• 1st reduces intensity by 1/2.
• 2nd reduces according to Malus’ Law




                                        10
Polarization by
Scattering and Reflection
 • Light scattered at
   90 degrees is
   100% polarized.




                            11
               Summary
• all waves diffract & interfere
• coherence required for strong effect
• path difference determines interference for
  double-slits and thin-films
• light polarization & polarizing filters




                                            12
       thin-film interference
• causes certain colors to become more or
  less intense.
• Ex. oil on water, soap bubble, coatings
• 1+2 constructive or destructive




                                            13
           Reflection phase
• off higher index: ½ wavelength phase
  change (Ex. ray 1 in diagram)
• off lower index: 0 (Ex. ray 2 in diagram)




                                              14
    Thin Film Problem Solving
 2t                      1, 2, 3,         Bright 
          (0 or 2 ) *   1 1 1
                 1
                                                   
 film                    2 , 1 2 , 2 2 , Dark 



    * 0: no reflection phase changes (or two)
    * ½: one reflection phase change



                                                       15
Ex. t = 100nm. Light wavelength = 400nm in
water.
            Ray 1 has reflection phase
            change, Ray 2 does not. (= ½)



            2t                  2(100 nm) 1
                     (0 or ) 
                          1
                                         2
            film         2
                                 400 nm

                               1  1 1
                                2   2      Bright


                                                    16
         diffraction grating
• N = #lines/cm,
• Spacing: d = 1/N

d sin q  n    n  0,1, 2, ... Bright

• Video demo


                                         17
     Single Slit Diffraction


a = Slit-Width




  (a ≠ d)
                               18
                waves
• pass through another wave undisturbed
• diffract video demo
• Refract and reflect




                                          19

								
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