Transcript Chapter 28
Chapter 29 Light Waves In this chapter we will study Huygens’ Principle Diffraction Interference Polarization Holography 1. HUYGENS' PRINCIPLE Every point on a wave front can be regarded as a new source of wavelets, which combine to produce the next wave front, whose points are sources of further wavelets, and so on. Huygen’s Principle can be used to explain the propagation of wave fronts involved in reflection, refraction, and diffraction. Consider several points on the wave front to be sources of secondary wavelets. . . . . . . . . . . . . . . 2. DIFFRACTION The bending of light that passes around an obstacle or through a narrow slit, causing the light to spread and to produce light and dark fringes. Demo - Laser, diffraction accessories, wire mesh screen, and rainbow disks Water waves on the surface of water If the wavelength of water waves are small compared to the size of an ocean vessel, the vessel will cast a “shadow.” For the same waves a post will not cast a “shadow.” Long wavelengths bend a great deal around small objects. Because of diffraction AM radio waves travel farther than FM radio waves. Microscopy diffraction limit One cannot see details of objects that are approximately the same size as or smaller than the wavelength of the illuminating light. 3. INTERFERENCE Slide - Interference Transparencies 3. INTERFERENCE Slide - Interference Transparencies Demo - Finger slit interference Demo - Single-color thin film interference Demo - Optical flats and sodium lamp Demo - Newton's rings and phase reversal Interference Colors by Reflection from Thin Films Iridescence - the phenomenon of seeing colors by interference in thin films. Demo - Soap bubbles and pipe Demo - Peacock feathers Demo – Thin film plates Example - Coated photographic lenses Interferometers measure small distances. Phase Reversal upon reflection Air Thin Film t Air Path Difference = 2t = nl for destructive interference n = 1,2,3,4,…… You see the complement of whichever color is Thus t = n(l/2) destructively interfered with. 4. POLARIZATION Light waves are transverse. This is verified by polarization. Most light you see is unpolarized meaning that it is composed of waves with vibrational planes in all directions. Common sources of light are not polarized. Polaroid Crystals - one axis direction absorbs, one transmits. Polarized glare occurs on reflection from nonmetallic surfaces. COLORS BY TRANSMISSION THROUGH POLARIZING MATERIALS Video - Crossed Polaroids and Crystal Video - Polarizers and Stress Demo - Polaroids and accessories Polaroid sunglasses worn in a normal viewing position will eliminate (a) vertically (b) horizontally (c) all polarized glare. Two polaroids that have their polarization axes at 45o to each other will still allow light to pass through. (a) True (b) False Three-Dimensional Viewing Stereoscopic viewers Slide - Stereogram 5. HOLOGRAPHY Hologram means whole message. Demo - Reflection hologram Demo - Transmission hologram No lenses are used for imaging in the creation or the viewing of a hologram. Object and source both illuminate all of the photographic plate. The light used to make the hologram must be coherent. A hologram is an interference pattern. It is best to use coherent light when viewing a hologram. A hologram can be divided. One gets a magnified holographic image by viewing a hologram with a longer l of light than was used in creating the hologram.