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Interference Diffraction and Lasers

Interference Diffraction and Lasers

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Interference Diffraction and Lasers

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  1. Interference Diffraction and Lasers Chapter 15

  2. Interference of Light • Superposition of 2 identical wavetrains traveling in same or opposite directions • Property of all waves, longitudinal and transverse, including light • First shown by Thomas Young in 1801 by passing monochromatic light through two narrow slits • Results in areas of increased and decreased intensity

  3. Interference patterns • Interference with monochromatic light produces alternate light and dark bands called fringes • Bright fringes are caused by constructive interference, with waves in phase • Dark fringes are caused by destructive interference, waves out of phase

  4. Double Slit Interference • Light passing through two narrow slits diffracts and overlaps producing interference pattern on screen • For constructive interference, path difference equals whole-number multiple of wavelength: • For destructive interference path difference must be odd number of half wavelengths:

  5. Double Slit Interference

  6. Thin Film Interference • Light reflects from top and bottom surface of thin, transparent film • Each reflection travels different distance, so interference results, depending on thickness of film • Some wavelengths are canceled, some reinforced

  7. Thin Film Interference • Result is swirling rainbow effect seen in soap bubbles, gasoline on water, etc. • When distance difference is 1/2 l, (3/2, 5/2, etc.) constructive interference occurs - phase is reversed in one reflected ray • When distance difference is 1l, (2, 3, etc.) destructive interference occurs, color is canceled, comp. color seen

  8. Uses of Interference • Regular surfaces produce regular interference patterns • Used to check measurements, tolerances, etc. • Interferometer uses interference patterns to make precise distance measurements

  9. Huygen’s Principle • Waves spreading from point source are made of many overlapping small waves • Every point on the wave is a point source of secondary waves • Explains diffraction Christian Huygens

  10. Diffraction • Spreading of a wave into area beyond barrier or small opening • Causes wave to bend • Occurs in all waves • More pronounced when obstruction or opening is small compared to wavelength

  11. Diffraction • Long e-m waves easily diffracted around buildings, hills, etc. (AM radio) • Visible light diffracted by objects around 10-7 m; determines limit of optical microscope • Electron beam has shorter wavelength so electron microscopes can resolve much smaller objects

  12. Diffraction of Light • 1816: Fresnel explained diffraction with interference • Diffraction through double slit or single slit both cause interference, slightly different pattern

  13. Diffraction Gratings • Transmission grating: transparent film with many evenly spaced fine lines • Reflection grating: reflective surface with many evenly spaced grooves • Diffraction angle depends on wavelength so light is dispersed showing spectrum • Interference causes spectrum to be repeated

  14. Diffraction Calculations • Grating constant (d) is distance between lines on the grating • n is number of spectrum • qn = Diffraction angle of each spectrum • For first order spectrum, (n = 1) l = d sinq • For any other spectrum, l = (d sinqn)/n

  15. Lasers • Stands for: Light Amplified by Stimulated Emission of Radiation • Emit coherent light: same direction, frequency, phase • Ordinary light sources are incoherent: chaotic, mixed frequencies, no phase relationship, all directions

  16. Spontaneous Emission • Energy is absorbed by atoms causing electrons to move to higher energy levels • Atom is in excited state • Electrons fall back to normal levels emitting photons of light • Atom returns to ground state

  17. Stimulated Emission • Excited states are usually very unstable • Many materials can be brought to slightly stable (metastable) energized state • Controlled energy input can create a population inversion where more atoms are in metastable excited state than in ground state.

  18. Stimulated Emission • Spontaneous emission of one photon causes avalanche of identical photons through chain reaction • All photons have same energy and frequency, so light is monochromatic

  19. Laser Construction • Lasing cavity is shaped for resonance at desired frequency; emissions at other frequencies quickly die out • Energy input from electricity or light flashes excites lasing medium • Mirrors at each end reflect laser light back through medium amplifying beam

  20. Laser Construction • Mirror at one end weakly silvered so beam can escape when strong enough • Some lasers pulse, some continuous • Many lasing materials discovered, gases, liquids, dyes, solids, semiconductors, crystals, etc.

  21. Holograms • Produced by interference of coherent light, gives 3-D image • Beam is split with one half going directly to film, other half reflects off subject • Since beams travel different distances, interference occurs • Interference pattern produced on film