Wave Optics

1
Wave Optics

• Chapter 24

2
Introduction to Optics

• Geometrical optics
• Reflection
• Refraction
• Wave optics
• Interference
• Diffraction
• Polarization

3
Thin Film Interference

• What causes the brilliant colors that you see
  reflected from oil or gasoline films on water or
  from the surface of soap bubbles?

4
Interference in Nature

• What causes butterflies and peacocks to have
  color?

5
Types of Interference

• Can you name two types of interference?
• Constructive interference
• Destructive interference

6
Conditions For Light Wave Interference

• Two conditions for interference
• Coherent source
• Waves are in phase
• Monochromatic
• Identical wavelengths
• Lasers are ideal light sources.

7
Youngs Double Slit Interference

• Two slits serve as a pair of coherent light
  sources.
• Fringes are produced on a screen.
• Bright lines (maxima)
• Waves arrive in phase.
• Dark lines (minima)
• Waves arrive out of phase.
• 24.1a, 259, 260

8
Path Differences

• Understanding the concept of path differences is
  critical to understanding thin film interference.
• Antenna signals
• 251, 257, 258,

9

• Path difference ( )
• Equations for constructive interference in double
  slits
• d is the slit separation
• 24.3a, 24.4

10
An Important Equation

• If and
• Then

11

• When working with double slits, we are concerned
  with the location of the BRIGHT fringes.

12
Change Of Phase Due To Reflection

• There is a 180o phase change when a wave reflects
  from the boundary of a more dense material.
• 24.6, 261

13

• There is no phase change when a wave reflects
  from the boundary of a less dense material.

14
(No Transcript)
15
(No Transcript)
16
Examples Involving Thin Film Interference

• Oil or gasoline on water
• Soap bubbles
• Coatings on camera lenses
• Colors produced by peacock feathers
• Colors in butterfly wings
• Blue eyed people

17
Light Interference in Soap Films
18
Interference In Thin Films

• Film thickness (t)
• Index of refraction (n)
• Equation for wavelength in the material
• ln is the wavelength in the given material
  with an index of refraction of n

19
Destructive Interference

• Equation for destructive interference

20
Constructive Interference

• Equation for constructive interference

21

• If there is a phase reversal at the second
  boundary, you must switch the equations for
  constructive and destructive interference

22
Newtons Rings

• Circular fringes caused by constructive and
  destructive interference
• Newtons Rings are used to check lenses for
  imperfections
• 24.37

23
(No Transcript)
24
Using Interference to Read CDs and DVDs

• CDs and DVDs provide high density storage of
  text, graphics, sound, and movies
• Dual-layer DVDs
• Blu-Ray technology (Sony)
• Competing technologies?

25
HD DVD vs. Blu-Ray
26

• The data is stored digitally as a series of zeros
  and ones.
• These are read by reflecting laser light from the
  shiny surface.
• There are pits (read as ones) and land areas
  (read as zeros).
• 268

27

• Strong reflections are read as zeros.
• constructive interference

28

• Weak reflections are read as ones.
• destructive interference

29

• The pit depth is made to be one quarter of the
  wavelength of the laser light in the plastic.

30
Photodetectors

• A photodetector is used to convert the
  reflections into an electronic string of ones and
  zeros.

31
CDs and DVDs

• Standard CDs use infrared lasers. (? 780 nm)
• Standard DVDs use red lasers. (? 650 nm)
• Blu-Ray DVDs use blue lasers.
• (? 405 nm)

32
Improving CDs and DVDs

• Shorter wavelengths allow us to store more
  information on a disk.
• CD (0.7 GB)
• DVD (4.7 GB)
• DVD Dual Layer (9.4 GB)
• Blu-Ray DVD (25 GB)

33
Questions

• 1, 2, 5, 6, 8
• Pg. 816

34
Double Slit-Diffraction
35
Diffraction of Light waves

• Youngs double slit experiment combined with
  Huygens Principle can be used to explain the
  diffraction of light waves.
• Diffraction of laser beams
• 24.13

36
Diffraction

• 3 parts of a diffraction pattern
• Central maximum
• Secondary maxima
• Minima
• 263, 259

37
Fresnel Diffraction

• The diffraction pattern for a penny
• There is a Fresnel bright spot in the center
• This is inconsistent with what you might expect
  from geometric optics
• 72

38
Single-Slit Diffraction

• According to Huygens Principle, light from one
  portion of a single slit can interfere with light
  from another portion.

39

• When working with single slits, we are concerned
  with the location of the dark fringes.

40
Fraunhofer Diffraction

• The Fraunhofer diffraction pattern for a
  single-slit has two characteristics.
• A wide bright central region
• Weaker maxima on both sides of a bright central
  maximum
• 24.16a b

41
Destructive Interference

• The equation for destructive interference in
  single slits
• a is the width of the slit

42
The Diffraction Grating

• A diffraction grating contains many, equally
  spaced parallel slits.
• There are several thousand lines per cm.
• The slit spacing is (d)
• 24.20

43

• Diffraction gratings are used for analyzing light
  sources.

44

• Diffraction gratings produce the brightest and
  sharpest maxima.

45
(No Transcript)
46
Locating the Maxima

• The equation for locating the maxima for a
  diffraction grating
• m is the order number

47
The Diffraction Grating Spectrometer

• Diffraction angles may be measured in order to
  calculate wavelength.
• 24.21

48
Diffraction Grating Applications

• CD and DVD drives use diffraction gratings for
  tracking.
• CDs, DVDs
• Reading/Writing/Rewriting
• Holograms
• Symbols on credit cards
• 262

49
Polarization Of Light Waves

• Polarization proves electromagnetic waves are
  transverse.
• The electric and magnetic field vectors are at
  right angles to each other.
• 24.24

50
Unpolarized Light

• All orientations of the electric field vector are
  possible.
• 24.26

51
Linearly Polarized Light

• Only one orientation of the electric field vector
  is possible.
• The waves are said to be plane polarized or just
  polarized

52
Malus Law

• The equation for the intensity of polarized light
  (Malus Law)
• is the intensity before passing through the
  analyzer. It is 0.5 times the original intensity.

53
How to Polarize Light Waves

• There are three processes by which
  electromagnetic waves may be linearly polarized.
• Selective absorption
• Reflection
• Scattering

54

• 1) Selective absorption
• Light is polarized as it passes through a
  polarizing material (polarizer).
• The transmission axis is usually vertical or
  horizontal.
• An analyzer may be used to identify polarization

55
Double Refraction

• Calcite displays double refraction
• Each image is oppositely polarized

56
(No Transcript)
57

• 2) Reflection from a shiny surface
• The reflected and refracted beams are both
  polarized.
• Polarizing angle (qp)
• According to Brewsters Law
• 245

58
Polarization by Reflection

• Light is polarized when reflected from
• Water
• Glass
• Metal
• Snow

59
Polarization in Photography

• Polarizing filters are used in photography.
• The filter attaches to the lens.

60
Polaroid Sunglasses

• Polarizing filters are also used in sunglasses.

61

• Polaroid sunglasses are vertically polarized
  because reflections are horizontally polarized.

62

• 3) Scattering
• Light passing through the earths atmosphere

63

• Light is also scattered as it passes through
  liquid crystals in iPods, iPhones, calculators,
  laptop computer screens, digital data projectors.

64

• Light is scattered as it passes through some
  transparent materials like Karo syrup.

65
Polarization in Nature

• Honeybees make use of polarization in the sky to
  tell direction.

66
Optical Activity

• Optical ability is the capacity of a substance to
  rotate the plane of polarization.
• It may occur in materials under stress

67
Polarization Applications

• Liquid crystal displays are used in
• Watches
• Cell Phones
• Calculators
• Laptop computer screens
• Flat screen televisions
• Others?

68
Questions

• 9, 11 14
• Pg. 816