Wave Optics Chapter 24
Introduction to Optics • Geometrical optics • Reflection • Refraction • Wave optics • Interference • Diffraction • Polarization
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?
Interference in Nature • What causes butterflies and peacocks to have color?
Types of Interference • Can you name two types of interference? • Constructive interference • Destructive interference
Conditions For Light Wave Interference • Two conditions for interference • Coherent source • Waves are in phase • Monochromatic • Identical wavelengths • Lasers are ideal light sources.
Young’s 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
Path Differences • Understanding the concept of path differences is critical to understanding thin film interference. • Antenna signals 251, 257, 258,
Path difference ( ) • Equations for constructive interference in double slits: d is the slit separation 24.3a, 24.4
An Important Equation If and Then:
When working with double slits, we are concerned with the location of the BRIGHT fringes.
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
There is no phase change when a wave reflects from the boundary of a less dense material.
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
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
Destructive Interference • Equation for destructive interference
Constructive Interference • Equation for constructive interference
If there is a phase reversal at the second boundary, you must switch the equations for constructive and destructive interference
Newton’s Rings • Circular fringes caused by constructive and destructive interference • Newton’s Rings are used to check lenses for imperfections 24.37
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?
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
Strong reflections are read as zeros. • constructive interference
Weak reflections are read as ones. • destructive interference
The pit depth is made to be one quarter of the wavelength of the laser light in the plastic.
Photodetectors • A photodetector is used to convert the reflections into an electronic string of ones and zeros.
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)
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)
Questions 1, 2, 5, 6, 8 Pg. 816
Diffraction of Light waves • Young’s double slit experiment combined with Huygens’ Principle can be used to explain the diffraction of light waves. • Diffraction of laser beams 24.13
Diffraction • 3 parts of a diffraction pattern • Central maximum • Secondary maxima • Minima 263, 259
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
Single-Slit Diffraction • According to Huygens’ Principle, light from one portion of a single slit can interfere with light from another portion.
When working with single slits, we are concerned with the location of the dark fringes.
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
Destructive Interference • The equation for destructive interference in single slits: ais the width of the slit
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
Locating the Maxima • The equation for locating the maxima for a diffraction grating: m is the order number
The Diffraction Grating Spectrometer • Diffraction angles may be measured in order to calculate wavelength. 24.21
Diffraction Grating Applications • CD and DVD drives use diffraction gratings for tracking. • CDs, DVDs • Reading/Writing/Rewriting • Holograms • Symbols on credit cards 262
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
Unpolarized Light • All orientations of the electric field vector are possible. 24.26