Babinet’s Principle is actually the principle used in optics. According to its definition,
“When the field behind a screen with an opening is added to the field of a complementary structure, the sum is equal to the field when there is no screen”
The images clearly explain the principle. In all the regions, which are non-collinear with the beam, the above two screens, in figure 1 & 2, produce the same diffraction pattern.
Consider a light source and a conducting plane (field) with an aperture before a screen. The light does not pass through the opaque area but passes through the aperture.
Consider the light source and a conducting plane of the size of the aperture in the previous case, being held against the screen. The light does not pass through the plane but does through the remaining portion.
Combine these two conducting planes of both the cases and put before the light source. The screen is not placed to observe the resultant combination. The effect of the screen gets nullified.
When an infinite conducting sheet is made a rectangular cut and the fields are excited in the aperture (which is called as a slot), such a thing can be termed as a Slot Antenna.
An Infinite plane conducting screen is taken and pierced with apertures of desired shape and size and this will be the screen of a slot antenna. Another screen is considered interchanging the places of aperture and screen area which is the complementary screen.
These two screens are said to be complementary as they result in the complete infinite metal screen. Now, this becomes the slot antenna which is an example of an aperture antenna. The terminal impedance is quite desirable for the radiation.