ydft = fft(y);
ydft = ydft(1:floor(length(ydft)/2)+1);
[maxval,I] = max(abs(ydft));
freq = 0:fs/length(y):fs/2;
fprintf('Maximum at %2.3f Hz\n.', freq(I));
You would have been better with:
t=0:ts:n/f-ts;
y=15*(1-cos(2*pi*f*t/n)).*cos(2*pi*f*t);
ydft = fft(y);
ydft = ydft(1:length(ydft)/2+1);
[maxval,I] = max(abs(ydft));
freq = 0:fs/length(y):fs/2;
fprintf('Maximum at %2.3f Hz\n.', freq(I));
Due to the spacing of the frequencies in the DFT. As far as the wavelength, you have not told us the propagation speed, which you have to in order to determine wavelength. If we assume that it is the speed of light, then
lambda = 3e8/freq(I);
fprintf('Wavelength is %3.2f meters.\n',lambda)
