1 / 24

Interference Diffraction and Lasers

Interference Diffraction and Lasers. Chapter 15. Interference of Light. Superposition of 2 identical wavetrains traveling in same or opposite directions Property of all waves, longitudinal and transverse, including light

ova
Download Presentation

Interference Diffraction and Lasers

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related