Conceptually, light at frequency f n = f 0 + n f r is doubled to 2 f n = 2 f 0 + 2 n f r , and mixed with light at the very similar frequency f 2 n = f 0 + 2 n f r to produces a beat signal at frequency 2 f n − − f 2 n = f 0 . In practice, this is not done with a single frequency f n but with a range of n values, but the effect is the same