meanings of the high and low frequency

Question

I have a question. What does "high frequency" and "low frequency" mean in Fourier analysis or maybe in more general sense? Thanks in advance!

Answer 1

$\cos(n\,t)$ and $\sin(n\,t)$ are periodic functions of period $2\,\pi/n$ and frequency $n/(2\,\pi)$. "High frequency" refers to terms in the Fourier series development of a function with large $n$, and "low frequency" to those with small $n$. A high frequency signal will have only terms with large $n$; similarly a low frequency signal. How large or small may depend on context.