An optical device having an optical axis (y-y) and comprising a source of light; a reflector which is associated with the source of light in order to form a light beam; and a rotary assembly which is designed to intercept the light beam, and can be rotated around an axis of rotation between first and second distinctive lighting positions. The rotary assembly comprises at least first and second shields associated respectively with the first and second lighting positions, in order to create a cut-off of the light beam. The first and second shields each comprise at least one ridge. The rotary assembly additionally is designed to permit progressive transition of the lighting between the first and second distinctive lighting positions.