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