Unit 1- Class 5- Liquid Crystals (Effects on liquid crystals)

ElelctroOptical effects in liquid crystals
Nematic liquid crystals are the simplest forms with rod like molecular structure and align themselves spontaneously along the director.
Dielectric anisotropy ( defined as the difference between the dielectric constant parallel and perpendicular to the director. The optical anisotropy ( is defined as the difference between the refractive indices parallel and perpendicular to the director. These two properties are important for the electro-optic effects in liquid crystals.


Effects of electric fields
The director in a liquid crystal is free to point in any direction. But when a film of liquid crystals is placed between two plates of certain materials, the director is forced to point along a perpendicular direction. When a thin film of liquid crystal is sandwiched between two glass plates, the molecules close to the glass surface are forced to orient themselves parallel to the surface of the glass sheets.
In the absence of an electric field (below a threshold value) the directors at other layers are also aligned parallel to the surfaces giving a homogeneous texture. But when an electric field is applied perpendicular to the molecules near the center of the crystal orient themselves along the applied field. The deformity begins at a threshold vale of the strength of the applied field and increases with increase in the strength of the field. This deformity brings about a significant change in the optical characteristics of the liquid crystals and are made use in liquid crystal displays


Effects of light
When light is incident on two crossed polarizes no light emerges as the light coming out of the first polarizer is completely absorbed by the second polarizer and hence appears dark. When a pair of crossed polarizers is filled with a twisted nematic liquid crystal having a positive dielectric anisotropy, the twisted structure acts like a wave-guide and gradually rotates the plane of polarization of light by 90o . Hence a linearly polarized light incident on the cell emerges linearly polarized but in an orthogonal direction resulting in a bright appearance. The 90o twist in the cell is lost when a sufficiently strong electric field is applied to the cell. Hence the cell appears to dark between the two crossed polarizers. However the following conditions should be met to see the elelctrooptic effects
(i) The plane of polarization of the incident light should be parallel or perpendicular at the surface of the cell
(ii) The product of the optical anisotropy, and the pitch P should be greater than the wavelength of the incident light

in the next class we will discuss the applications of liquid crystals.