### 1.1. The best wavelength for a particular polariscope?
The *Polarizer* (the circular polarizer between the light source and the sample) and the *Analyzer* (the circular polarizer between the camera and the sample) of a polariscope ([transmissive](/transmission-photoelasticity) or [reflective](/reflection-photoelasticity)) contain same type of quarter-wave plate. This quarter-wave plate sets a particular wavelength that works best for this polariscope to probe the photoelastic fringes. A different light wavelength would create larger noise, since passing the quarter-wave plates twice no longer gives a $`\pi`$ phase difference between the two perpendicular components of the light. Thus using different wavelengths can not give a perfect dark field for this polariscope.
The *Polarizer* (the circular polarizer between the light source and the sample) and the *Analyzer* (the circular polarizer between the camera and the sample) of a polariscope ([transmissive](/transmission-photoelasticity) or [reflective](/reflection-photoelasticity)) contain same type of quarter-wave plate. This quarter-wave plate sets a particular wavelength that works best for this polariscope to probe the photoelastic fringes. A different light wavelength would create larger noise, since passing the quarter-wave plates twice no longer gives a \(\pi\) phase difference between the two perpendicular components of the light. Thus using different wavelengths can not give a perfect dark field for this polariscope.
