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:

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 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

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 qn = Diffraction angle of each spectrum For first order spectrum, (n = 1) l = d sinq For any other spectrum, l = (d sinqn)/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