An optical element is provided for generating interfacial waves by means of electro wetting. The optical element comprises a fluid chamber with at least one side wall and an optical axis. Furthermore the fluid chamber includes a first fluid (A) and a second fluid (B) which are separated by an interface, the fluids (A, B) being immiscible. A first electro wetting electrode and a second electro wetting electrode are provided, the first electro wetting electrode is separated from the first fluid (A) and the second fluid (B) by a fluid contact layer. The second electro wetting electrode is arranged to act on the first fluid (A). The selected standing or running waves can be formed by providing selected voltages to the first and the second electro wetting electrodes respectively.