Light Scattering

1
Light Scattering

• Rayleigh Scattering Mie Scattering

2
Theory

• Characteristics of Polystyrene and Sulphur
  Nanoparticles.
• -- Non absorbing
• -- Refractive index is a weak function of
  wavelength.
• How does light interact with objects?
• -- Reflection (light deviated from its original
  path)
• -- Refraction
• -- Diffraction
• -- Absorption (light absorbed and converted to
  heat)

3
Theory Cont

• A collimated light source is the most basic tool
  for nanoparticle work. Often called a Tyndall
  beam.
• The Tyndall effect is the scattered path of light
  observed in the suspension. Examples Milk,
  smoke, Lake Thunderbird.
• Scattering Plane
• -- The scattering plane is defined by the two
  rays involved, the source-particle ray and the
  particle-observer ray.
• -- The scattering plane is determined by
  observation, it is not fixed in space. For
  example, if the observer moves, the scattering
  plane will move with the observer.

4
Theory Cont.

• Theory of Rayleigh
• -- Particles are treated as electric dipole.
• Results
• -- I ?1/ ?4 (only true if the refractive index
  is a weak function of ?, i.e. not a metal.)
• -- I ?r6
• -- scattered light at 90 is linearly polarized
  perpendicular to the scattering plane.

Horizontal Source Polarization
Verticle Source Polarization
5
Theory Cont

• Mie Scattering
• -- Absorption and Scattering by a Sphere.
• -- Multipole expansion (EM modes of a sphere)
• -- electric dipole.
• -- magnetic dipole, electric quadrupole.
• -- magnetic quadrupole, electric octupole.
• If d lt ?/20 then only the first term (dipole) is
  needed. In this limiting case, Mies theory
  reduces to Rayleighs theory.
• Efficiency factors Qsca, Qabs, Qext
• Plot Qext vs ? for the extinction spectra
• Qsca and Qabs vs ? show their contribution to
  Qext.
• Intensity for perpendicular and parallel
  polarized light
• Plot I vs ? for the angular intensity dependence
  for each polarization.

6
Objective

• Learn about Scattering plane
• The Polarization of Rayleigh Scattering
• Mie Scattering
• Angular Dependence

7
Procedure

• Rayleigh scattering
• Using a light source and polarization lens we
  observed the way light rays are polarized through
  rayleigh scattering in different solutions
  Silica SOL, Sulfur SOL and Fine Sulfur particles,
  by shining the light source through the solutions
• Shine light source though solutions in a dark
  room
• Place polarized lens in path of light source to
  observe polarization effects of scattered light
  in the scattering plane and outside of scattering
  plane, I.E. view from top (90 degrees) and other
  angles of observation

8
Higher Order Tyndall Spectra (HOTS)

• Using the same procedure to observe the Rayleigh
  scattering effects, observe the different colors
  associated with the scattered light and observe
  angle dependency
• Note the number of orders in each sample, one
  order is one color change from red to green

9
Scattering Angle

• Using the Helium Neon laser apparatus (wavelength
  543.5 mm) the laser beam was shone through our
  samples of polystyrene latex
• The samples were placed at an angle on the
  observation stage to avoid multiple reflection of
  the laser beam in the same area of the solution
• Note the minimum intensity zones in the scattered
  light by observing in the horizontal plane and
  recording the angle these minima occur. This was
  done by observing from about 35 degrees to 145
  degrees from the laser beam in the scattering
  plane. See picture

Laser
Sample
Observation stage with angle measurement site
10
Optical Microscopy

• Using an optical microscope in dark field
  observation mode, observe the nanoparticles in
  each of the three samples
• Note their movement and size (each sample
  resembles a night sky filled with stars, the
  particles can be seen but not studied in detail)

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12
Results Rayleigh Scattering

• Silica SOL
• With the vertical polarization lens in place we
  noticed there is no angular dependence in the
  scattering plane
• By using a second polarization lens at a 90
  degree angle we verified that the light is
  polarized perpendicular to the scattering plane
• Sulphur SOL
• Using the same techniques we noted the HOTS
  followed slightly different result patterns,
  there was significantly more forward scattering
  which caused the scattered light to blend
  together and appear simply white toward larger
  scattering angles in the scattering plane
• Polystyrene latex
• By observing each of the three samples and noting
  their order using the HOTS phenomenon we ranked
  the sample by particle size from smallest to
  largest as follows Sample C, Sample B, Sample A
  (This initial ranking seemed to be correct
  according to our scattering angle experiment
  data)

13
Observation
Sample Gaurav Trevor Ye
A 50 49 85
74 73 98
98 100 105
124
B 54 54 50
96 96 98
C 141 141 141
14
Results Scattering Angle

• Sample C
• No minimum were found in this sample although the
  intensity significantly decreased as the viewing
  angle decreased, this result is consistant with
  the Mie plot data
• Estimated particle size ?lt240 nm
• Sample B
• Minima recorded at scattering angles of 54 and 96
  degrees
• Estimated particle size ? 600nm
• Sample A
• Minima recorded at 50, 74 and 98 degrees
• Estimated particle size ? 1060 nm

15
Mie plot results
Sample C Particle size lt 240nm
Sample A Particle size 1060nm
50
Intensity decreases as angle increases
74
98
Sample B Particle size 600nm
54
96
16
Experiment discussion

• Experimentally determined particle size
• Sample A Particle size ? 1060 nm
• Sample B Particle size ? 600 nm
• Sample C Particle size ? lt 240 nm
• Error and procedure improvement suggestions
• Lab results were recorded by human observation of
  three different lab technicians, humans always
  make mistakes
• These results could be improved my taking more
  measurements and averaging results
• Mono-dispersed vs. Poly-dispersed HOTS
• A more highly dispersed sample would appear more
  milky under observation, that is to say the
  light spectrum would be blended together and
  appear more like white light instead a showing
  distinct wavelengths