We already covered Snell’s law in the section “Wave,” but we provide an alternative derivation using calculus in the second article. To appreciate Fourier transformations, you must know linear algebra well, but it is simple enough to understand without it. I included the article on Fourier transformations here because it is not usually covered in freshman mathematics while its construction is interesting. For more motivations behind Fourier transformations, please read “What is Fourier series?” listed in the section “Trigonometric functions.” Actually, the whole book “Who is Fourier?” treats this topic, along with all its prerequisites such as trigonometric functions, complex numbers, calculus, and vector. I highly recommend the book. “Expectation values in quantum field theory (1)” prepares you to understand Feynman diagram.