Spark image


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.

Student investigation
Photoelastic stress patterns may be observed very easily as follows.
Place a piece of Polaroid on the top of an overhead projector and blank off the rest of the transmitted light. Put the object under test, such as a protractor or clear plastic ruler, on top of the Polaroid and place a second piece of Polaroid on top of the specimen. The projection on the screen will show the photoelastic patterns clearly.

Alternatively a slide projector may be used, one piece of Polaroid being placed in the slide holder and the second piece taped on the front of the projector lens. The test object can then be hung between the two. Using this method differing loads may be applied to the bottom of the specimen and the resulting change in the stress pattern observed.

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 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:

Specific rotation (s) = [Lc]/d

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 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
© Keith Gibbs 2011