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
- Results in areas of increased and decreased
intensity
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
- 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
- When obstruction has dimensions comparable
to wavelength, light spreads into area behind
obstruction
- Can use slit, fine wire, sharp edge, pinhole
- 1816: Fresnel explained diffraction with
interference
- double slit or single slit both cause interference
Diffraction Gratings
- Transmission grating: transparent film with
many evenly spaced fine lines
- Reflection grating: reflective surface with
many evenly spaced grooves
- Interference of light diffracted by each
line or groove creates multiple images (spectra)
Diffraction Calculations
- Grating constant (d) is distance between
lines
- n is number of spectrum
- θn = Diffraction angle of each spectrum
- For first order spectrum, l = d sinθ
- For any other spectrum, l = (d sinθn)/n
- Wave oscillations confined to single plane
- Only transverse waves, (all e-m waves)
- Normal light emissions unpolarized, plane
of vibration random
- Can be polarized by interaction with matter
- Electrical component of e-m waves interacts
with matter
- Represent electrical oscillation by vector
Selective Absorption
- Certain crystals absorb component of light,
transmit its perpendicular component - dichroism
- Result is light with all oscillations in
same plane
- 1935 Land develops method to make polarizing
filters, starts Polaroid corp.
- sunglasses, photo filters
Polarization by Reflection
- Smooth surfaces reflect component of light
parallel to surface; perpendicular component
absorbed or refracted
- Maximum polarization occurs at Brewster�s
angle (polarizing angle)
- To reduce glare, Polaroid sunglasses have
vertically aligned filters
Polarization by Refraction
- Certain crystals (e.g. calcite) have different
index of refraction for perpendicular components
of light wave
- One obeys Snell�s law, one doesn�t
- Result is separation of components into 2
polarized beams or images
Polarization by Scattering
- Small particles in transparent medium will
scatter light, cause partial polarization
- Size of particle determines frequency of
light affected
Structural Colors
- Some colors due to scattering of light and
polarization: no pigments for colors
- Blue light absorbed and scattered by air
molecules, causes blue sky
- Red sunsets due to path of light from setting
sun passing through more atmosphere, more
blue scattering
- Blue jay feathers, peacocks blue eyes
Uses of Polarized light
- Perpendicular polarized light beams can�t
interfere
- If beam is rotated, interference results
- Certain plastics become doubly refractive
when stressed; Can detect stress points with
polarized light and detector
Optical Rotation
- Optically active substances rotate plane
of incident light
- Amount of rotation measured by polarimeter
- e.g. sugar - content can be measured by how
much polarized light is rotated