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Dr. Quantum General Physics 2Light as a Wave1. General Physics 2Light as a Wave2 The Nature of Light When studying geometric optics, we used a ray model.

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Presentation on theme: "Dr. Quantum General Physics 2Light as a Wave1. General Physics 2Light as a Wave2 The Nature of Light When studying geometric optics, we used a ray model."— Presentation transcript:

1 Dr. Quantum General Physics 2Light as a Wave1

2 General Physics 2Light as a Wave2 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...

3 General Physics 2Light as a Wave3 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

4 General Physics 2Light as a Wave4 Huygen’s Principle

5 General Physics 2Light as a Wave5 Diffraction Huygen’s Principle is useful for understanding diffraction - the bending of waves behind obstacles into the shadow region

6 General Physics 2Light as a Wave6 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

7 General Physics 2Light as a Wave7 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

8 General Physics 2Light as a Wave8 what you see on the screen:

9 General Physics 2Light as a Wave9 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?

10 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!)

11 General Physics 2Light as a Wave11 Visible Spectrum

12 General Physics 2Light as a Wave12 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)

13 General Physics 2Light as a Wave13 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

14 General Physics 2Light as a Wave14 Diffraction by a Disk Diffracted light interferes constructively at center of shadow requires a point source of monochromatic light (e.g. laser)

15 General Physics 2Light as a Wave15 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

16 General Physics 2Light as a Wave16 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

17 General Physics 2Light as a Wave17 Diffraction Grating double slit versus diffraction grating for multi-wavelength light

18 General Physics 2Light as a Wave18 Interference by Thin Films Produces rings of constructive/destructive interference

19 General Physics 2Light as a Wave19 Thin-Film Interference: Examples Soap bubbles Oil on water Beetles Butterflies

20 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 2Light as a Wave20

21 General Physics 2Light as a Wave21 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?

22 General Physics 2Light as a Wave22 Emission Tubes – real and PhET 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 Element


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