Cross-polarized light

Polarized light microscopy can mean any of a number of optical microscopy techniques involving polarized light. Simple techniques include illumination of the sample with polarized light. Directly transmitted light can, optionally, be blocked with a polariser orientated at 90 degrees to the illumination. More complex microscopy techniques which take advantage of polarized light include differential interference contrast microscopy and interference reflection microscopy. Scientists will often use a device called a polarizing plate to convert natural light into polarized light.

These illumination techniques are most commonly used on birefringent samples where the polarized light interacts strongly with the sample and so generating contrast with the background. Polarized light microscopy is used extensively in optical mineralogy.

As polarised light passes through a birefringent sample, the phase difference between the fast and slow directions varies with the thickness, and wavelength of light used. The optical path difference (o.p.d.) is defined as ${\displaystyle {o.p.d.}=\Delta \,n\cdot t}$, where t is the thickness of the sample.

This then leads to a phase difference between the light passing in the two vibration directions of ${\displaystyle \delta \,=2\pi \,(\Delta \,n\cdot t/\lambda \,)}$. For example, if the optical path difference is ${\displaystyle \lambda \,/2}$, then the phase difference will be ${\displaystyle \pi }$, and so the polarisation will be perpendicular to the original, resulting in all of the light passing through the analyser for crossed polars. If the optical path difference is ${\displaystyle {n}\cdot \lambda \,}$, then the phase difference will be ${\displaystyle 2{n}\cdot \pi }$, and so the polarisation will be parallel to the original. This means that no light will be able to pass through the analyser which it is now perpendicular to.

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