FIR filter and the relation between Sample rate and cut-off frequency

I have problem to unterstand the logic of the cut-off frequency of the FIR filter. Suppose my sample rate is 2MHz and I want to design a lowpass filter with a cut-off frequency 100Hz. What will be $\omega_c$ below:

LowpassFilter[data,$\omega_c$,n]

and what is the limit on kernel "n"?

• Under Details and Options in the docs: "With SampleRate->r, the cutoff frequency Subscript[[Omega], c] should be between 0 and r*[Pi]." So I always just specify sample rate then use the cutoff I want times Pi. LowpassFilter[data,100*\[Pi],n,SampleRate->2*10^6] will filter a 2MHz signal at 100Hz. An audio example is also shown in the docs. There isn't any info in the docs, but it looks like the limit on the kernel is Length@data, it doesn't complain if n is larger, but it doesn't change the answer. – N.J.Evans Sep 28 '17 at 11:17