**5. Electromagnetic Duality**

The electric field induced by an electric dipole has similar properties to the magnetic field created by a magnetic dipole. Such similarities can be considered as an emergen<sup>t</sup> form of *electromagnetic duality*. To correctly understand electromagnetic duality, we need to clarify the difference between electric and magnetic fields. To this end, we utilize inner- and outer-oriented vectors called *polar* and *axial* vectors, respectively [78]. By using these concepts, we accurately formulate electromagnetic duality, while the role of orientation of the space in electromagnetic duality is elucidated. The analogy between electromagnetic duality and Keller–Dykhne duality is also discussed.
