Question

In a polarized optical microscope, under a crossed polarized system, a sample of an anisotropic crystal is placed, and the interference color will be changed if the state is rotated. At this time, if the polarizing plate is removed and the natural light is directly irradiated to the crystal, can the change of interference color also be observed when the stage is rotated? Why? Please give the explaination.

Answer 1

Given that

In a polarized optical microscope, under a crossed polarized system, a sample of an anisotropic crystal is placed, and the interference color will be changed if the state is rotated.

Specified Condition : At this time, if the polarizing plate is removed and the natural light is directly irradiated to the crystal

can the change of interference color also be observed when the stage is rotated? No

there is a destructive interference will happen

explanation lines :

The polarized light microscope is designed to observe and photograph specimens that are visible primarily due to their optically anisotropic character.

In order to accomplish this task, the microscope must be equipped with both a polarizer, positioned in the light path somewhere before the specimen,

and an analyzer (a second polarizer), placed in the optical pathway between the objective rear aperture and the observation tubes or camera port.

Image contrast arises from the interaction of plane-polarized light with a birefringent (or doubly-refracting) specimen to produce two individual wave components that are each polarized in mutually perpendicular planes.

The velocities of these components are different and vary with the propagation direction through the specimen. After exiting the specimen, the light components become out of phase with each other, but are recombined with constructive and destructive interference when they pass through the analyzer.

When an anisotropic specimen is brought into focus and rotated through 360 degrees on a circular polarized light microscope stage, it will sequentially appear bright and dark (extinct), depending upon the rotation position.

When the specimen long axis is oriented at a 45-degree angle to the polarizer axis, the maximum degree of brightness will be achieved, and the greatest degree of extinction will be observed when the two axes coincide.

During rotation over a range of 360 degrees, specimen visibility will oscillate between bright and dark four times, in 90-degree increments.

This is due to the fact that when polarized light impacts the birefringent specimen with a vibration direction parallel to the optical axis, the illumination vibrations will coincide with the principal axis of the specimen and it will appear isotropic (dark or extinct).

If the specimen orientation is altered by 45 degrees, incident light rays will be resolved by the specimen into ordinary and extraordinary components, which are then united in the analyzer to yield interference patterns.

Because interference only occurs when polarized light rays have an identical vibration direction,

the maximum birefringence is observed when the angle between the specimen principal plane and the illumination permitted vibrational direction overlap.

Interference between the recombining white light rays in the analyzer vibration plane often produces a spectrum of color, which is due to residual complementary colors arising from destructive interference of white light.

The colors observed under illumination with white light in the microscope eyepiece can be utilized to quantitatively draw conclusions about path differences and specimen thickness values when the refractive indices of the specimen are known.

an anisotropic crystal under crossed polars (analyzer inserted) the crystal is extinct when either of the two privileged directions in the crystal are lined up parallel to the polarizing direction of the microscope.

This is because when the privileged directions are parallel to the polarizer, the crystal does not change the polarization direction and the light will thus be vibrating perpendicular to the analyzer.

When the privileged directions are not parallel to the polarizer some light is transmitted by the analyzer and this light shows a color, called the interference color.

When polarized light enters an anisotropic crystal from the given condition and neither of the privileged directions in the crystal are parallel to the polarizer,

the light is broken up into two mutually perpendicular polarized waves that travel through the crystal. ( destructive interference )

Further, when the two waves emerge from the top of the crystal, they may not be in the same position in their vibration as when they entered the crystal

(i.e. they may be out of phase, depending on the path difference acquired while in the crystal).