Question

write a note on phase contrast Microscope??

Answer 1

In recent years, remarkable advances have been made in the study of living cells (unstained) by the development of special optical techniques such as phase contrast and interference microscopy.

The biological specimens are highly transparent to visible light and they cause phase changes in transmitted radiations.



The phase contrast microscope has the same resolving power as the ordinary light microscope but it permits visualization of different parts of the cell due to differences in their refractive index (Refractive index is defined as the ratio of the velocity of light in a vacuum to its velocity in a transmitting medium).

Because light is transmitted through a structure at a velocity inversely proportional to the refractive index of the structure, light waves emerging from structures with different refractive index will be out of phase with one another.

The phase contrast microscope is able to convert these differences in phase to differences in light intensity, producing an image with good contrast. The phase-contrast microscope utilizes interference between two beams of light.

In the phase contrast microscope, the small phase differences are intensified. The most lateral light passing through the objective lens of the microscope is advanced or retarded by an additional l/4th wavelength (1/4λ.) with respect to the central light passing through the medium around the object, by an annular phase plate that introduces a 1/4 wavelength variation in the back focal plane of the objective.