W.B.C.S. Notes On – Physics – Polarization Optical Activity.

WBCS পরীক্ষার নোট – পদার্থবিজ্ঞান – মেরুকরণ অপটিক্যাল ক্রিয়াকলাপ।

Optical activity, the ability of a substance to rotate the plane of polarization of a beam of light that is passed through it. (In plane-polarized light, the vibrations of the electric field are confined to a single plane.) The intensity of optical activity is expressed in terms of a quantity, called specific rotation, defined by an equation that relates the angle through which the plane is rotated, the length of the light path through the sample, and the density of the sample (or its concentration if it is present in a solution). Because the specific rotation depends upon the temperature and upon the wavelength of the light, these quantities also must be specified.Continue Reading W.B.C.S. Notes On – Physics – Polarization Optical Activity.

The rotation is assigned a positive value if it is clockwise with respect to an observer facing the light source, negative if counterclockwise. A substance with a positive specific rotation is described as dextrorotatory and denoted by the prefix d or (+); one with a negative specific rotation is levorotatory, designated by the prefix l or (-).

Optical activity was first observed in quartz crystals in 1811 by a French physicist, François Arago. Another French physicist, Jean-Baptiste Biot, found in 1815 that liquid solutions of tartaric acid or of sugar are optically active, as are liquid or vaporous turpentine. Louis Pasteur was the first to recognize that optical activity arises from the dissymmetric arrangement of atoms in the crystalline structures or in individual molecules of certain compounds.

Light interacts with matter. The interaction leads to an array of phenomena of which the most common are refraction (also called refringence) and reflection. These phenomena depend on the type of matter (e.g., metal or dielectric) and the properties of the light, such as color (= frequency), direction of propagation with respect to the material, and polarization. Any form of polarization dependence of the interaction between light and matter is called optical activity. This definition includes effects that occur without the presence of an applied magnetic or electric field (so-called natural optical activity) and polarization-dependent phenomena that occur upon placing matter in a magnetic or electric field (so-called magneto– and electro-optical activity). Two types of optical activity can be discerned: birefringence(also called double refraction) and dichroism. We define birefringence as the dependence of the speed of monochromatic light on polarization. Circular birefringence (CB) occurs when the speeds of left and right circular polarized light differ and linear birefringence (LB) when the speeds of two orthogonal linearly polarized light beams differ. CB is the most common phenomenon and gives rise to a rotation of the polarization plane of linearly polarized light (see below). In the case that these phenomena are caused under the influence of an applied magnetic field they are called magnetic circular birefringence(MCB) and magnetic linear birefringence (MLB). We define dichroism as the dependence of the absorption (or emission) of monochromatic light on polarization. Circular dichroism (CD) occurs when the absorptions (or emissions) of left and rightcircularly polarized light in matter differ and linear dichroism (LD) when the absorptions (emissions) of two orthogonal linearly polarized light beams differ. In the case that these phenomena are caused under the influence of an applied magnetic field they are called magnetic circular dichroism (MCD) and magnetic linear dichroism (MLD).

It depicts the time evolution of the oscillating electric field component of a circularly polarized monochromatic light beam at a fixed position in space. Can one tell from whether the light is left or right circularly polarized? The answer is no, one can’t! To know the sense of rotation, one needs to specify either the direction of light propagation (indicated by the k vector in  or the magnetic component of the electro-magnetic field (indicated in . From the reader’s point of view the sense of rotation of E is the same in all four diagrams in . However, if the light propagates toward the reader the polarization is right circular (R) and away from the reader the polarization is left circular (L) . If we consider the 90° angle between H and E in then we see that H rotates toward E in the case of right circular polarization and E rotates toward H in the case left circular polarization. These characterizations are intrinsic properties of light and independent of reader’s perspective. Just like the reader for telling left from right, a physical system must somehow take into account either the combination of E and k or of E and H in order to be optically active. In other words, one can discern two types of physical mechanism for explaining natural optical activity: E, k mechanisms and E, H mechanisms. The E, k mechanisms depend essentially on the spatial variation of the electro-magnetic wave along the direction k and the E, H mechanisms on an interplay of electric polarization and magnetic induction. A more detailed mechanism for either one will be specified further on. In the presence of an applied magnetic (or electric) field one should think of the physical system as “matter plus field”. So defined, the system provides a unique spatial direction of reference (namely the field) for unambiguously assigning a rotational sense to the rotating electric field E. The rotational sense defined with respect to the applied field is indicated by R’ or L’ in . The sense defined with respect of the field can either coincide or be opposite to the rotational sense of the radiation (the latter being defined with respect to k). Magneto-optical effects involving circular or elliptic radiation depend essentially on the distinction between R’ and L’ and not on that between L and R.

Our own publications are available at our webstore (click here).

For Guidance of WBCS (Exe.) Etc. Preliminary , Main Exam and Interview, Study Mat, Mock Test, Guided by WBCS Gr A Officers , Online and Classroom, Call 9674493673, or mail us at – mailus@wbcsmadeeasy.in
Visit our you tube channel WBCSMadeEasy™ You tube Channel
Please subscribe here to get all future updates on this post/page/category/website