Download Interference, Diffraction and Polarization

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: d (sin  )  m
• For destructive interference path difference
must be odd number of half wavelengths:
d (sin  )   m  12  
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 sinn)/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