Lecture handout including QS - Department of Materials Science ...

BH10 Course B: Materials for Devices BH10
Remember - the phase difference is dependent upon the birefringence of the sample, the wavelength
of the light, and the sample thickness:
δ
2π = Δn.t
λ
Look at a sample between crossed polars (polarizer ⊥ r analyzer):
• if section is isotropic (non-birefringent), no light will be transmitted through the analyzer.
• if sample is birefringent, and the optical path difference is an integral number of wavelengths,
Nλ, then there will be zero phase difference between the 2 components, the plane of polarization
will be unchanged, and hence no light of wavelength λ will be transmitted.
⇒ if a birefringent sample is illuminated with white (i.e. polychromatic) light, one wavelength of
that light will be lost through the condition described above. Therefore the light observed will
consist of the full optical spectrum minus that specific wavelength (the complementary colour).
The colour will depend upon the birefringence and the thickness of the sample:
The Michel-Levy chart
Very low retardation:
(very thin section, or very
low birefringence).
→
→
Move through higher
‘orders’ of colours as
o.p.d. = λ, 2λ, 3λ etc.
High retardation: several different wavelengths
are ‘lost’ (with different integer values, e.g.
o.p.d. = 2λ 2 = 3λ 1 ), & colours become washed out
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Retardation = optical path difference = Δn × t