Chapter 24

1. Chapter 24 The Wave Nature of Light

2. Diffraction • Isaac “Big Daddy Fig” Newton • Supporter of the particle theory of light • Christian Huygens (1629-1695) • Supporter of wave theory of light • Huygen’s Principle • Every point on a wave front can be thought of as a point source of new wavelets • The new wavefront envelops all the wavelets • It is tangent to all wavelets

3. Diffraction • Shadows • Waves can bend around corners into the “shadow zone” behind obstacles • This bending is called diffraction • Particularly noticeable when waves pass through small openings • Francesco Grimaldi (1618-1663) noted that the ray model of light could not explain shadows

4. Young’s Double Slit Experiment • Thomas Young (1773-1829) • Double Slit Experiment • Passed light from a point source, through a screen with two thin slits • Viewing screen on the other side showed several lines • Spacing of lines depended on color of light and width of slits • Monochromatic—light of a single wavelength

5. Young’s Double Slit Experiment • Particle theory prediction—two slits should yield two lines on the viewing screen • Wave theory prediction • Each slit would be a point source of new light waves (Huygen’s Principle) • Waves from both sources would overlap • Constructive interference—bright spots • Destructive interference—dark spots • Spacing of interference pattern depends on wavelength • Conclusion—different colors have different wavelengths

6. Double Slit Diffraction • Predicting the interference patterns • Constructive interference (bright spots) • dsin = m • Destructive interference (dark spots) • dsin = (m + ½) • d = distance between slits •  = angle between horizontal and line drawn from slits to bright spot •  = wavelength of light • m = order of interference spot—0, 1, 2, 3 . . .

7. Double slit Diffraction

8. Double slit Diffraction

9. Double Slit Diffraction • Problem solving (Double slit diffraction) • m = 0, indicates the center bright spot • m = 1, indicates the 1st spot to the left and right • Right triangle geometry using d, , x (distance from horizontal to spot), and L (distance from slits to viewing screen) • When <<1,  = sin  = tan  (in radians)

10. Double Slit Diffraction • Replacing the two slits with two small light sources would not yield the same pattern • Two slits make two point sources with a constant phase (timing) relationship, because they come from the same source • Coherent—waves that keep a constant phase relationship • Two lights would not start emitting at the same time, and emission timing could fluctuate • Incoherent—waves that have no fixed phase relationship

11. Double Slit Diffraction • With polychromatic light (multiple wavelengths), diffraction fringes contain a spectrum with all wavelengths separated • Single slit diffraction • Dsin = m (minima, dark spots) • D = width of slit • Differently shaped openings will produce differently shaped diffraction patterns • Discovery of double helix by Roslyn Franklin

12. Rosalind Franklin’s X-ray diffraction pattern of DNA

13. Spectroscopy • Spectroscope—device using a diffraction grating to accurately measure wavelengths of light • Diffraction gratings are rated in lines/m • Used to identify materials • Line spectrum—include only certain discrete wavelengths • emitted by heated or excited gases • Continuous spectrum—include a wide range of wavelengths • Emitted by heated solids and very dense gaseous objects

14. Spectroscopy • Absorption spectrum—continuous spectrum with certain distinct wavelengths missing • Atoms absorb light at the same frequencies at which they emit light • Used to identify materials that light reflects off of or passes through

15. Thin-film Interference • Rainbows seen in thin films of soap, oil, etc. • Reflection occurs at the top and bottom of the film • Reflected rays interfere with one another • Path difference between rays determine whether interference is constructive or destructive

16. Thin-film Interference

17. Thin-film Interference • A light wave traveling in one medium and reflected from the boundary of a medium with a larger index of refraction (n2 > n1) undergoes a 180° phase shift • If the reflecting medium has a smaller index of refraction (n2 < n1), no phase shift occurs • Normal situations (n0 < n1 < n2) • Air-oil-water • Air-coating-lens

18. Thin-film Interference

19. Thin-film Interference

20. Thin-film Interference • Constructive Interference (phase shift at both boundaries) • Path difference (twice the thickness of the film) is an exact multiple of the wavelength of light • Constructive Interference (phase shift at one boundary) • Path difference (twice the thickness of the film) is an odd multiple of half-wavelengths • Remember, the wave flips around

21. Thin-film Interference • Destructive Interference (phase shift at both boundaries) • Path difference (twice the thickness of the film) is an odd multiple of half-wavelengths • Destructive Interference (phase shift at one boundary) • Path difference (twice the thickness of the film) is an exact multiple of the wavelength of light • Remember, the wave flips around

22. Thin-film Interference • Nonreflective coatings for lenses • Phase shift occurs at both boundaries • The point is to use destructive interference to eliminate reflection

23. Polarization • Waves oscillate in particular geometrical planes • Unpolarized light—oscillations occur randomly in all directions • Polarized light—oscillations are restricted to a single plane

24. Polarization

25. Polarization

26. Polarization • Reflected light can be polarized • It is completely polarized when the reflected and refracted rays for a right angle • Polarizing angle (p) = incident angle that causes complete polarization

27. Polarization • Polarized light can be blocked by a second polarizer • If axes of polarization match up, it’ll pass through fine • If the axes of polarization are perpendicular, the polarized light will be completely blocked