A beam of ordinary light may be regarded as a bundle of electromagnetic waves vibrating in all directions perpendicular to the axis of the beam. When such a beam of light is made to pass through a specially cut crystal of certain minerals or sheet of special plastic polaroid or a crystal of Iceland spar, all vibrations except those in one plane are eliminated. This is called plane polarised light (Figure 2.7).

Figure 2.7 Different Forms of Polarised Light

When such a beam of plane polarised light is passed through a solution of optical isomer, if the plane polarised light is found to rotate to the left, it is known as laevorotation; if the plane polarised light rotates to an equal number of degrees to the right, it is known as dextrorotation. This phenomenon exhibited by the asymmetric compounds is called optical isomerism.