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## Wave Optics

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**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**Diffraction gratings produce the brightest and sharpest**maxima.**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