Antibiotic Manufacturing

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Antibiotic Manufacturing

Phase Contrast Microscope

When light is passed through a live biological specimen on a slide, no significant differences in absorption of light by different parts of the specimen can be seen unless the specimen is crowded by pigments such as chlorophyll or melanin.

This leads to images with little or no contrast. Staining preparations can provide the much needed contrast but staining

can lead to artifacts. Therefore, phase contrast microscopes are designed to provide contrast to live specimens to detect cell inclusions under natural conditions.

These microscopes are equipped with special interference optics to exploit phase differences in natural biological specimens to reveal their distinct identities.

The microscope has an annulus in the condenser that provides a hollow cone of light to brighten the specimen. Some light rays pass through the specimen without deviation (solid lines) while others deviate (dashed line). The deviated rays cause phase shifts because the light slows down with parts of the specimen, the extent of shift related to the thickness and the refractive index of a particular component or structure within a cell.

In many biological structures, the shift is about 90 degrees or one fourth of a wavelength. The objective lens of the microscope is equipped with a phase plate so designed that undeviated rays obtain an additional phase shift of 90 degrees over the deviated rays.

When both the deviated and undeviated rays reach the eye piece lens to form an image, the light rays are out of phase by 180 degrees (one half of a wavelength) producing destructive interference and causing the structures to appear dark against the bright background.

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