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Polarized light microscopy

Summary

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.[1]

Polarizing microscope operating principle
Depiction of internal organs of a larva via birefringence and polarized light microscopy

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.

History

Although the invention of the polarizing microscope is typically attributed to David Brewster around 1815, Brewster clearly acknowledges the priority of Henry Fox Talbot, who published his work in 1834.[2][3]

The Michel-Levy chart

Michel-Lévy interference colour chart issued by Zeiss Microscopy

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.

The Michel-Levy Chart (named after Auguste Michel-Lévy) arises when polarised white light is passed through a birefringent sample. If the sample is of uniform thickness, then only one specific wavelength will meet the condition described above, and be perpendicular to the direction of the analyser. This means that instead of polychromatic light being viewed at the analyser, one specific wavelength will have been removed. This information can be used in a number of ways:[citation needed]

• If the birefringence is known, then the thickness, t, of the sample can be determined
• If the thickness is known, then the birefringence of the sample can be determined

As the order of the optical path difference increases, then it is more likely that more wavelengths of light will be removed from the spectrum. This results in the appearance of the colour being "washed out", and it becomes more difficult to determine the properties of the sample. This, however, only occurs when the sample is relatively thick when compared to the wavelength of light.

References

1. ^ "Basics of Polarizing Microscopy" (PDF). Olympus. Archived (PDF) from the original on 15 December 2016. Retrieved 15 December 2016.
2. ^ David Brewster (2021). A Treatise on the Microscope. Thomas Allan. Retrieved 4 January 2021.
3. ^ William Henry Fox Talbot (1834). "XLIV. Experiments on light". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. 5 (29): 321–334. doi:10.1080/14786443408648474.