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PHYS 1442 – Section 004 Lecture #20

PHYS 1442 – Section 004 Lecture #20. Monday April 6, 2014 Dr . Andrew Brandt. Ch 24 Wave Nature of Light The Visible Spectrum and Dispersion Diffraction by a Single Slit or Disk Diffraction Grating. Announcements. a) HW9 on ch 23 due Tues 15 th

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PHYS 1442 – Section 004 Lecture #20

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  1. PHYS 1442 – Section 004 Lecture #20 Monday April 6, 2014 Dr. Andrew Brandt • Ch 24 Wave Nature of Light • The Visible Spectrum and Dispersion • Diffraction by a Single Slit or Disk • Diffraction Grating

  2. Announcements a) HW9 on ch 23 due Tues 15th b) HW10 on ch 24-25 due Tues 22nd c) Test on Weds. Apr. 23

  3. 24-1 Waves Versus Particles; Huygens’ Principle and Diffraction Huygens’principle: Every point on a wave front acts as a point source; the wavefront as it develops is tangent to their envelope

  4. 24-1 Waves Versus Particles; Huygens’ Principle and Diffraction Huygens’principle is consistent with diffraction:

  5. 24-2 Huygens’ Principleandthe Law of Refraction

  6. 24-2 Huygens’ Principle andthe Law of Refraction Huygens’principle can also explain the law of refraction. As the wavelets propagate from each point, they propagate more slowly in the medium of higher index of refraction. This leads to a bend in the wavefront and therefore in the ray.

  7. 24-2 Huygens’ Principle andthe Law of Refraction (24-1) The frequency of the light does not change, but the wavelength does as it travels into a new medium.

  8. 24-2 Huygens’ Principle andthe Law of Refraction Highway mirages are due to a gradually changing index of refraction in heated air.

  9. 24-3 Interference—Young’sDouble-SlitExperiment If light is a wave, interference effects will be seen, where one part of wavefront can interact with another part. One way to study this is to do a double-slit experiment:

  10. 24-3 Interference—Young’sDouble-SlitExperiment If light is a wave, there should be an interference pattern.

  11. 24-3 Interference—Young’sDouble-SlitExperiment The interference occurs because each point on the screen is not the same distance from both slits. Depending on the path length difference, the wave can interfere constructively (bright spot) or destructively (dark spot).

  12. 24-3 Interference—Young’sDouble-SlitExperiment (24-2a) (24-2b) We can use geometry to find the conditions for constructive and destructive interference:

  13. 24-3 Interference—Young’sDouble-SlitExperiment Between the maxima and the minima, the amplitude varies smoothly.

  14. 24-3 Interference—Young’sDouble-SlitExperiment Since the position of the maxima (except the central one) depends on wavelength, the first- and higher-order fringes contain a spectrum of colors.

  15. 24-4 The Visible Spectrum and Dispersion Wavelengths of visible light: 400 nm to 750 nm Shorter wavelengths are ultraviolet; longer are infrared

  16. 24-4 The Visible Spectrum and Dispersion The index of refraction of a material varies somewhat with the wavelength of the light.

  17. 24-4 The Visible Spectrum and Dispersion This variation in refractive index is why a prism will split visible light into a rainbow of colors.

  18. 24-4 The Visible Spectrum and Dispersion Atmospheric rainbows are created by dispersion in tiny drops of water.

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