4. An optical system is consist of one concave lens of focal length -f and one convex lens of focal length f as shown below. The object is placed at a distance 2f away from the concave lens. If we illuminate the object from left with a normally incident plane wave of wavelength, we would produce the Fourier transform of the object at plane P. Then, how much is the distance Z between plane P and the convex lens? Explain it clearly! Don't just give the answer!! (12%) Object 2 f I Z