Crossed Nicols
Definition
The configuration in which the polariser (below the condenser) and the analyser (above the objective) are oriented at 90 degrees to each other, extinguishing all transmitted light unless the specimen has altered the polarisation state.
- Configuration
- Polariser and analyser set 90 degrees apart
- Effect with no specimen
- Extinguishes all transmitted light
- Effect with specimen
- Light passes if the specimen alters polarisation
- Instrument
- Polarising microscope
Common questions
Why is a dark field the useful starting point in this configuration?+
With no specimen present, the field goes black because the analyser blocks all light that already passed the polariser at 90 degrees. Any brightness that then appears when a sample is placed on the stage must come from the specimen itself rotating the light's polarisation, which makes optically active or birefringent materials stand out clearly.
What kind of forensic materials show up under crossed Nicols when they would not under plain light?+
Birefringent materials such as many synthetic fibres, certain minerals, and some crystalline drug residues rotate polarised light and appear bright or show interference colours, while optically isotropic materials such as glass or amorphous plastics stay dark, helping distinguish between similar-looking trace materials.
Related terms
- Autofluorescence
- Native fluorescence of endogenous biological molecules (collagen, elastin, NADH, flavins) that produces background signal in fluorescence microscopy, potentially mimicking specific probe signals....
- Birefringence
- The difference between the highest and lowest refractive indices of an anisotropic mineral, seen as a characteristic interference colour between crossed polars....
- Conoscopic Illumination
- A convergent polarised beam mode in which a range of angles is passed through the specimen simultaneously, producing an interference figure (optical...
- DAPI
- 4',6-diamidino-2-phenylindole; a fluorescent dye that binds double-stranded DNA and emits blue (~460 nm) under UV excitation (~360 nm). Standard nuclear counterstain; used...
- Dichroic Mirror
- A spectrally selective beam splitter that reflects shorter (excitation) wavelengths at 45 degrees and transmits longer (emission) wavelengths, forming the core element...
- Epi-Fluorescence Illumination
- Fluorescence excitation through the objective lens from above the specimen, with the same objective collecting the emission. A dichroic mirror directs excitation...
- Extinction Angle
- The angle, measured in degrees on the rotating stage of a petrographic microscope, between a crystallographic reference direction (cleavage trace or crystal...
- Michel-Lévy Chart
- A birefringence-thickness diagram correlating retardation values with the interference colour seen at that retardation under white-light crossed polars. Standard reference for identifying...
- Photobleaching
- The irreversible photochemical destruction of a fluorophore by repeated excitation. Limits the observation time for low-copy forensic fluorescence samples. Mitigated by antifade...
- Retardation
- The path difference (in nm) introduced between the fast and slow ray components as they traverse an anisotropic specimen of given thickness...
- Stokes Shift
- The wavelength difference between the excitation peak and the emission peak of a fluorophore. A positive Stokes shift (emission at longer wavelength...