A switchable optical unit, capable of controlling a beam of radiation (b) passing through an optically active portion (8) of the unit, comprises a fluid chamber (10) including an electrically conductive liquid (18), which chamber comprises a pair of first electrodes (20,22) fixed to the chamber inner walls (12,14) at the position of the optically active portion and second electrode means (24) fixed to inner walls of the chamber outside the optically active portion and a third electrode (28) connected to the conductive liquid. By applying a voltage (V) to the first electrodes and the second electrode means, respectively the conductive liquid can be moved in and out the optically active portion so that the unit (1) is switched between a least two discrete states. |