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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 - PowerPoint PPT Presentation
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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
• First shown by Thomas Young in 1801 by passing monochromatic light through two narrow slits
• Results in areas of increased and decreased intensity
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
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:
(sin )d m
12(sin )d m
Double Slit Interference
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
Thin Film Interference• Result is swirling rainbow effect seen in soap
bubbles, gasoline on water, etc.• When distance difference is 1/2 , (3/2, 5/2,
etc.) constructive interference occurs - phase is reversed in one reflected ray
• When distance difference is 1, (2, 3, etc.) destructive interference occurs, color is canceled, comp. color seen
Uses of Interference
• Regular surfaces produce regular interference patterns
• Used to check measurements, tolerances, etc.
• Interferometer uses interference patterns to make precise distance measurements
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
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
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
Diffraction of Light
• 1816: Fresnel explained diffraction with interference
• Diffraction through double slit or single slit both cause interference, slightly different pattern
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
Diffraction Calculations
• Grating constant (d) is distance between lines on the grating
• n is number of spectrum
• n = Diffraction angle of each spectrum
• For first order spectrum, (n = 1) d sin
• For any other spectrum, d sinn)/n
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
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
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.
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
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
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.
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
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