Polarized Microscope Q.1 What does it mean for the light to be “Polarized” ? Natural sunlight and almost every other form of artificial illumination transmits light waves whose electric field vectors vibrate in all perpendicular planes with respect to the direction of propagation. A light wave that is vibrating in more than one plane is referred to as unpolarized light. If the electric field vectors are restricted to a single plane by filtration of the beam with specialized materials, then the light is referred to as plane or linearly polarized with respect to the direction of propagation, and all waves vibrating in a single plane are termed plane parallel or plane-polarized.

Q.2 What is meant by Plane and crossed polarized light? In the figure, the incident light electric field vectors are vibrating perpendicular to the direction of propagation in an equal distribution of all planes before encountering the first polarizer. Polarizer 1 is oriented vertically to the incident beam so it will pass only the waves that are vertical in the incident beam. The wave passing through polarizer 1 is subsequently blocked by polarizer 2 because the second polarizer is oriented horizontally with respect to the electric field vector in the light wave. The concept of using two polarizers oriented at right angles with respect to each other is commonly termed crossed polarization and is fundamental to the practice of polarized light microscopy.

Image contrast arises from the interaction of plane-polarized light with a birefringent 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. 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.

Optical Activity Many molecules have an interesting property in that they can rotate polarized light. Some drugs molecules can rotate or change the direction the light vibrates, so if we place a container of the drug solution in the light path, between the two filters (polarizers) in the figure , then in order to look “light,” or “dark”, the second filter will have to be rotated differently to make up for how much the drug molecule rotates the light. Drugs can be identified by which direction and how much it rotates the light. For example, glucose rotates polarized light to the right so it’s also known as dextrose. Fructose rotates polarized light to the left, so it’s also known as levulose.

Origins of optical activity. An electronic transition is the result of the movement of charges when a molecule is exposed to light. The electronic transition has an associated magnetic transition that is perpendicular to it. The energy of a transition depends on the electric dipole moment and the magnetic dipole moment induced by the action of light on the electrons in the molecule. The rotational strength of a transition is the imaginary part of the dot product of the electric dipole induced by the light and the magnetic dipole induced by the light. If a molecule has a plane or center of symmetry either the sum of all the induced electric and magnetic dipoles is zero, or the vectors representing the magnetic and electric dipoles are perpendicular to one another. The result is that there is no optical activity since the cosine of 90 deg. equals 0. There are several cases showing asymmetry in the molecule and in these cases the molecule is optically active.