LTI: Linear TIme Invariant
==This we know that can be done when the wavelength () or the field, which we are describing, it isn’t actually field or something like this, is much larger than the size, the min size of the device we are trading ()==.
==You have to operate at the frequency (), which is below a maximum frequency (), which is dictated by this size and the speed of the way we are trading. So we consider that we can do this==, we take this assumption for granted