Illusion optics is an electromagnetic theory that can change the optical appearance of an object to be exactly like that of another virtual object, i.e. an illusion, such as turning the look of an apple into that of a banana. Invisibility is a special case of illusion optics, which turns objects into illusions of free space. The concept and numerical proof of illusion optics was proposed in 2009 based on transformation optics in the field of metamaterials.[1] It is a scientific disproof of the idiom 'Seeing is Believing'.[2]
Illusion optics proves that the optical responses or properties of a space containing any objects can be changed to be exactly those of a virtual space but containing arbitrary virtual objects (illusions) by using a passive illusion optics device composed of materials or metamaterials with specific parameters and shape. For example, a dielectric spoon was numerically shown to exhibit the scattering properties of a metallic cup by using an illusion optics device in the seminal paper.[1] Such illusion effects do not rely on the direction and form of incident waves. However, due to dispersion limitation of specific material parameters, the functionality of illusion optics device only works in a narrow band of frequency.