Leica Polarization Microscopes 2015 Polarization microscopy General Polarized light can be used to analyze materials that show the Phenomenon of Birefringence: • Crystals (Minerals) • Liquid Crystals • Polymers • Biological Samples • Drugs • Isotrope material with stress birefringence (Glass) • Liquids (flow birefringence) Basics 1. White light 1. White light Description of light waves: • • • Amplitude Wave length l Velocity of propagation => Brightness/Intensity => Colour => c c A l 2. Polarizer 1. White light 2. Polarizer One wavelength as example! Description of light waves: • • • • Amplitude Wave length Velocity of propagation Situation of polarisation Ex,Ey Ex Ey z 3. Sample [Birefringence] 2. Polarizer 1. White light 3. Birefringence 3. Birefringence The velocity of light change, when the light wave enters a material with a different refraction index c __ n= V c = light speed (in vacuum) V = velocity of light in the medium Material (isotropic) Air Water Glycerin Immersion Oil Glass Flint Zircon Diamond Lead Sulfide Refractive Index (n) 1.0003 1.33 1.47 1.518 1.52 1.66 1.92 2.42 3.91 3. Birefringence Beam path of ordinary and extraordinary waves e o e o Wave fronts of ordinary and extraordinary waves c __ no = V1 c __ ne = V2 V2 V1 (modified after Raith & Raase, 2009) 3. Birefringence OK…, well…, fine… But were are the colors??? 3. Birefringence Sample with birefringence G = l/2 • Beam is split off in two with perpendicular vibration directions. • Due to different refractive indices the waves travel with different speeds through the material. • The two waves leave the sample with an offset. (phase difference/ retardation) nz nx (Raith & Raase, 2009) Polarized light 3. Birefringence G phase difference/ ordinary ray retardation Ordinary and extraordinary ray show a phase difference G Phase difference is dependent on: ne and no extraordinary ray The difference of ne and no (n) is called birefringence! as well as from thickness of the sample (d) d d x (ne – no) = G Polarizer This is the color! By inserting the analyzer the phase difference becomes visible! 4. Analyzer 3. Birefringence 2. Polarizer 1. White light 4. Interference Dimensions of one light wave: • • • • Amplitude Wave length l Velocity of propagation Situation of polarization => Brightness/Intensity => Color => c => Ex,Ey EY c A more than one wave...? z Ex l z 4. Interference Dimensions of one light wave: • • • • Amplitude Wave length l Velocity of propagation Situation of polarization => Brightness/Intensity => Color => c => Ex,Ey EY more than one wave...? • phase shift/ retardation z Ex 4. Interference When does interference occure? Superposition of light waves, with same velocity and same oscillation plane (coherency). The nature of the resulting wave depends on the phase shift (retardation) of the interfering waves. EY Situation 1 constructive interference = 0, 1l, 2l ... Wave fronts in-phase • • Addition of the Amplitudes Higher intensity z Ex 4. Interference Wave fronts dephased with different Amplitudes Situation 2 destructive interference = lowering of the intensity =½l EY z Ex 4. Interference Wave fronts dephased and same Amplitudes Situation 3 EY destructive interference = extinction of resulting wave! =½l z Ex 4. Interference constructive interference by the analyzer Analyzer (Raith & Raase, 2009) 4. Interference destructive interference by the analyzer constructive or destructive interference (Raith & Raase, 2009) 50 The sum of constructive or destructive interference for all wavelength result in the 0 25 Michel-Lévy-chart G [nm] Intensity characteristic interference colors! phase difference G [nm] (Raith & Raase, 2009) Michel-Levi-chart G n Phase difference Birefringence n and corresponding mineral Thickness of sample in µm maximum interference colour Application examples: 1.determination of an unknown mineral d= must be given (25mm) maximum interference colour G= can be determent with the microscope 2. Thickness determination n= is given by the material G= can be determent with the microscope n = G d G d= n
