Fourier theory is pretty complicated mathematically. But there are some beautifully simple holistic concepts behind Fourier theory which are relatively easy to explain intuitively. There are other sites on the web that can give you the mathematical formulation of the Fourier transform. I will present only the basic intuitive insights here, as applied to spatial imagery.
M. Trodden. (1998)cite arxiv:hep-ph/9803479Comment: 113 pages, 10 figures, uses RevTeX. References added, several sections slightly expanded. To appear in Reviews of Modern Physics, October 1999.
M. Cheng, J. Duncan, und J. Harvey. (2012)cite arxiv:1204.2779Comment: 124 pages, 75 tables; updated references and minor editing in v.2; minor editing and two new theorems in v.3.