Photoelasticity
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.
The Kerr effect
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.
Optical activity
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 16
o for red light and
about 47
o for violet at 20
oC.
The liquid crystals used in calculator
displays, digital watches and lap top computer screens are also optically active. They 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 polarimeter
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.
Summary of the development of the theory of
light
1637 Descartes' Dioptrique
1657 Permat's Principle
1664 Hooke's Micrographia
1665 Grimaldi discovers diffraction
1670
Bartholinus discovers the double refraction of Iceland spar
1676 Romer
measures the velocity of light
1690 Huygens' Traite de La Lumiere
1704 Newton's
Opticks
1770 Euler's Dioptrica
1801-3 Young's papers on interference
1850
Foucault shows experimentally that the velocity of light in water is less than that in air
1865 Maxwell's electromagnetic theory of light
1887 Michelson-Morley
experiment
1901 Planck's quantum theory
1905 Einstein's special theory of
relativity and the explanation of the photoelectric effect
1922 De Broglie's theory of
electron waves
1925 Development of quantum mechanics
