# Meaning of the frequencies vector using fft

I'm so confused, and it would be nice if someone could clear my head, or at least direct me to suitable references.

Assume that I have two gaussian pulses with the same bandwidth in time, but for 2 different wavelengths. I'm going to handle them separatly- 2 separate graphs of one gaussian in time.

Both graphs are the same because they have the same bandwidth. Then, I use fft on each graph. Of course I'm going to have the same graphs after the fft- because I had the same graphs before.

Now, I want to creat the frequencies axis for each signal. In order to do that, I use the exact direction from matlab. df=1/T, T=N·dt. So the frequencies axis are also the same!

But how can it be if f=c/lambda? I'm talking about two different wave length, how is it possible that the both have the same spectrum of frequencies?

Further more, I don't understand what is the different between F and df and why is F=N·df. I do understand that T is the time in which I sample, dt=T/N. but that's it.