To provide an optical apparatus which controls an interface state to change a focal length by using an optical element having a container sealing first liquid that is conductive or polarized and second liquid that does not mutually mix with the first liquid with their interface in a predetermined form and electrodes provided in the container and of which optical characteristics change according to change of interface form due to application of voltage to the electrodes, and in particular an optical apparatus that controls a duty ratio of alternating current voltage applied to said electrodes for changing said interface form.