Any arrangement that cannot be superimposed with its mirror image by translation or rotation in three dimensions has extrinsic 3d chirality. Extrinsic means that the chirality is a consequence of the arrangement of different components, rather than an intrinsic property of the components themself. For example, the propagation direction of a beam of light through an achiral crystal (or metamaterial) can form an experimental arrangement that is different from its mirror image. In particular, oblique incidence onto any planar structure that does not possess two-fold rotational symmetry results in a 3D-chiral experimental arrangement, except for the special case when the structure has a line of mirror symmetry in the plane of incidence. Bunn predicted in 1945 that extrinsic 3d chirality would cause optical activity and the effect was later detected in liquid crystals. Extrinsic 3d chirality causes large optical activity and linear conversion dichroism in metamaterials. These effects are inherently tuneable by changing the relative orientation of incident wave and material. Both extrinsic 3d chirality and the resulting optical activity are reversed for opposite angles of incidence.