Dr. Quantum General Physics 2 Light as a Wave 1
The Nature of Light When studying geometric optics, we used a ray model to describe the behavior of light. A wave model of light is necessary to describe phenomena such as: interference diffraction A particle model of light is necessary to describe phenomena observed in modern physics, for example, the interaction between light and atoms. We ll get back to this later... General Physics 2 Light as a Wave 2
Wave Nature of Light Christian Huygens (1629-1695) contemporary of Newton developed wave theory of light Huygen s Principle Every point on a wave front can be considered as a source of tiny wavelets that spread out in the forward direction at the speed of the wave itself. The new wave front is the envelope of all the wavelets - tangent to all of them General Physics 2 Light as a Wave 3
Huygen s Principle General Physics 2 Light as a Wave 4
Diffraction Huygen s Principle is useful for understanding diffraction - the bending of waves behind obstacles into the shadow region General Physics 2 Light as a Wave 5
Interference Thomas Young (1773-1829) definitively (at least temporarily) demonstrates wave nature of light Young s Double-Slit Experiment coherent light passes through 2 slits, S 1 and S 2 light from S 1 and S 2 then interferes and pattern of dark and light spots is observed on the screen General Physics 2 Light as a Wave 6
Interference Constructive interference occurs when d sin θ = m λ, m = 0,1,2,... m = order Destructive interference occurs when d sin θ = (m + 1/2) λ, m = 0,1,2,... Source must be coherent waves at S1 and S2 are in-phase General Physics 2 Light as a Wave 7
what you see on the screen: General Physics 2 Light as a Wave 8
Think-Pair-Share Monochromatic light falling on two slits 0.016 mm apart produces the fifth-order fringe at an 8.8 degree angle. What is the wavelength of the light used? General Physics 2 Light as a Wave 9
Pair Problem Light of wavelength 680 nm falls on two slits and produces an interference pattern in which the fourth-order fringe is 38 mm from the central fringe on a screen 2.0 m away. What is the separation of the two slits? (Hint: tan θ = θ for small angles, and angles must be in radians!) General Physics 2 Light as a Wave 10
Visible Spectrum General Physics 2 Light as a Wave 11
Dispersion Index of refraction varies with wavelength of light As a result, white light is separated into component colors by a prism or by water (rainbow) General Physics 2 Light as a Wave 12
Dispersion & Rainbow red is bent the least red light reaches observer s eye from higher water droplets violet is bent the most violet light reaches observer s eye from lower water droplets General Physics 2 Light as a Wave 13
Diffraction by a Disk Diffracted light interferes constructively at center of shadow requires a point source of monochromatic light (e.g. laser) General Physics 2 Light as a Wave 14
Diffraction by a Single Slit position of minima D sin θ = m λ m = 1, 2, 3,... for m=1, theta gives 1/2 width of central maximum Motivation for making large diameter telescopes General Physics 2 Light as a Wave 15
Diffraction Grating a large number of equally spaced parallel slits same relation as double-slit d sin θ = m λ m = 0, 1, 2,... produces sharper and narrower interference patterns that double slit General Physics 2 Light as a Wave 16
Diffraction Grating double slit versus diffraction grating for multi-wavelength light General Physics 2 Light as a Wave 17
Interference by Thin Films Produces rings of constructive/destructive interference General Physics 2 Light as a Wave 18
Thin-Film Interference: Examples Oil on water Soap bubbles Beetles Butterflies General Physics 2 Light as a Wave 19
Find the diffraction grating spacing Align the laser and the diffraction grating Measure the length and height of peaks to determine the angle Use the diffraction equation to calculate the diffraction grating spacing. The laser wavelength is 630 nm General Physics 2 Light as a Wave 20
PhET Conceptual Question Use PhET simulation to answer the following questions http://phet.colorado.edu/en/simulation/wave-interference What happens to the interference pattern if the wavelength of light is increased from 500 nm to 700 nm? What happens instead if the wavelength stays at 500 nm but the slits are moved farther apart? General Physics 2 Light as a Wave 21
Emission Tubes real and PhET Element Look at several emission tubes using diffraction gratings & sketch spectrum Foundation of spectroscopy, a technique used in numerous scientific applications Compare with PhET simulation http://phet.colorado.edu/en/simulation/discharge-lamps General Physics 2 Light as a Wave 22