Iceland spar has a curious property called birefringence for influencing the light passing through.
The rectangular tank is filled with a solution of an enantiomer.
The light can be represented by wave trains formed by two vectors (electric field and magnetic field) alternately vibrating.
Natural light consists of many wave trains vibrating in random directions. Through the Nicol ( polarizer ) pass only those that vibrate in a plane ( plane polarization). Then the light is said to be polarized .
If the plane of polarization is first horizontal, the light is visible for an eye located at the tank level. Turning the Nicol by 90 o, the line of sight is located in the plane of polarization which makes the light invisible. (Compare what you see in a box, whose lid is at eye level!)