Some materials, including glass, celluloid, Bakelite and some other
plastics, become doubly refracting when subjected to stress. If a piece of such material is placed
between two crossed Polaraids the stress patterns can be observed. Different colours of light are
affected differently and some very beautiful effects can be obtained. Plastic models of components
such as gears, turbine blades and hooks can be made and the stress patterns in them observed to
check their design.
These patterns can be seen in car windscreens. The patterns stored in
them are due to the stresses produced during their manufacture.
In 1875 Kerr discovered that glass becomes doubly refracting when subjected to an intense electric field. It was later found that many liquids (nitrobenzene is one example) also showed this effect, the ordinary ray being in the direction of the field and the extraordinary ray perpendicular to the field. The effect follows the variation of the field very closely in nitrobenzene, disappearing within one nanosecond of the field being removed.
Some materials can rotate the plane of
polarisation of light as it passes through them Those that rotate it in a left-handed direction are
called laevorotatory and those that rotate it in a right-handed direction dextro-rotatory. They are
said to be optically active. The rotation produced is roughly proportional to the inverse square of
the wavelength. A plate of quartz 1 mm thick produces a rotation of 16o for red light and
about 47o for violet at 20 oC.
The liquid crystals used in calculator
displays, digital watches and lap top computer screens are also optically active. The amount of
rotation in these crystals can also be altered by applying an electric field between the two faces of
the screen and this is how the display is turned from bright to dark.
Some liquids, such as
sugar or turpentine and solutions of tartaric acid are optically active. The amount of rotation (?) is
found to be proportional to:
(a) the length of the liquid column L, and
(b) the
concentration of the solution c.
We define a quantity known as the specific rotation of a
solution (s) by the formula:
The specific rotation of a given liquid may be found using a polarimeter as shown in Figure 2. The two polaroids are adjusted to give a minimum light intensity, and the scale reading noted. A measured length of solution of known concentration is then placed in the inner tube and the polaroids readjusted to regain a minimum and the scale is read again. The rotation of the plane of polarization of the light by the solution may then be found from the difference in the two scale readings.