 # General Physics 2Light as a Wave1 The Nature of Light When studying geometric optics, we used a ray model to describe the behavior of light. A wave model.

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General Physics 2Light as a Wave1 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 2Light as a Wave2 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 2Light as a Wave3 Huygen’s Principle

General Physics 2Light as a Wave4 Diffraction Huygen’s Principle is useful for understanding diffraction - the bending of waves behind obstacles into the shadow region

General Physics 2Light as a Wave5 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 2Light as a Wave6 Interference Constructive interference occurs when d sin  = m, m = 0,1,2,... m = order Destructive interference occurs when d sin  = (m + 1/2), m = 0,1,2,... Source must be coherent waves at S1 and S2 are in-phase

General Physics 2Light as a Wave7 what you see on the screen:

General Physics 2Light as a Wave8 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 2Light as a Wave9 Conceptual Question 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 2Light as a Wave10 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  =  for small angles, and angles must be in radians!)

General Physics 2Light as a Wave11 Diffraction by a Disk Diffracted light interferes constructively at center of shadow requires a point source of monochromatic light (e.g. laser)

General Physics 2Light as a Wave12 Diffraction by a Single Slit D sin  = m m = 1, 2, 3,... position of minima for m=1, theta gives 1/2 width of central maximum Motivation for making large diameter telescopes

General Physics 2Light as a Wave13 Diffraction Grating a large number of equally spaced parallel slits same relation as double-slit d sin  = m m = 0, 1, 2,... produces sharper and narrower interference patterns that double slit

General Physics 2Light as a Wave14 Diffraction Grating (L) double slit versus diffraction grating for multi-wavelength light

General Physics 2Light as a Wave15 Emission Tubes Look at several emission tubes using diffraction gratings & sketch spectrum Foundation of spectroscopy, a technique used in numerous scientific applications Element

General Physics 2Light as a Wave16 Interference by Thin Films Produces rings of constructive/destructive interference

General Physics 2Light as a Wave17 Thin-Film Interference: Examples Soap bubbles Oil on water Beetles Butterflies

Group Problems Q2B.6, Q2S.9 General Physics 2Light as a Wave18

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