Electromagnetic spectrum

1

• Electromagnetic spectrum visible portion
• Violet (400 nm) ? Red (700 nm)
• White ROYGBV
• (can be separated by dispersion)

2
Electromagnetic waves (visible light, X-rays,
etc.)

• Wave Theory
• Wavelength l Distance between peaks
• Frequency no. of waves/sec to pass a given
  point (hertz)
• frequency v/l , v velocity

f
3
Pertinent Units

• Wavelength nm (mµ) 10-9m
• old units, Å 10-8m
• Frequency hz (cycles/sec)
• Energy units (at the atomic scale) are also
  sometimes used. Energy in eV ??, where ? is
  Planck,s constant. 1Å is equivalent to
  1.24x10-24eV.

4
Polarization

• Non-polarized (usual) light
• Each photon vibrates as a wave form in a single
  plane. Vibration refers to the rise and fall of
  the sine wave.
• Light beam is composed of numerous photons, each
  vibrating in a different plane.
• Vibration occurs in all directions
  perpendicular to propagation direction

5
Polarization
Plane of interface surface of crystal Plane of
incidence T-N-U-V-M-S-O Plane of
incidence contains incident, reflected and
refracted ray paths as well angles i, l and r
6

• Refraction
• Incident ray and reflected ray
• 1) ? of incidence i ? of reflection r
• 2) coplanar in the plane of incidence
• Refracted ray
• 1) Slower in water or glass
• 2) ?r (refraction) ? ? i (incidence)
• Depends on D v

Incident
Reflected
i
r
air
water
r
Refracted
7
Index of refraction

• For a substance x
• nx vvacuum/vx
• nair 1.0003
• light is slower in water, glass, crystals
• Is nwater greater or less than 1?
• Larger n associated with slower v !
• Snells Law (1621)
• ni sin i nr sin r
• for two known media (air/water),
• sin i/sin r nr / ni const.
• So can predict angle change (or use it to
  determine nr)

8
Polarization
incoming ray is non-polarized
reflected and refracted rays both become polarized

• Note symbolization used
• Reflected ray polarized perpendicular to the
  plane of incidence.
• Refracted ray polarized in plane of incidence.
  

9
Interference
Fig 7-1 Bloss, Optical Crystallography, MSA

• A Particles in phase if displaced from rest
  position by same amount in same direction
• a1 - a2 - a3 are all in phase
• b1 - b2 - b3 are also all in phase (but not with
  a1)
• particles perfectly out of phase
  equal-but-opposite displacement
• b1 and c1 are not perfectly out of phase

10
Interference
Fig 7-1 Bloss, Optical Crystallography, MSA

• path difference (D) distance between any 2
  points on a wave form
• usually expressed as xl
• D between any 2 points in phase il (iany
  integer)
• D between any 2 points perfectly out of phase
  ((2i1)/2) l/2