Sir, Consider a resonance column which is in unison with a tuning fork of particular frequency. Now by varying the length of the air column, resonance is obtained at different lengths. How is this possible? I have this doubt because each length of air column vibrates with a particular frequency. So when the length of air column is changed, the frequency also changes isn’t it? My question is even after the change in frequency, how is resonance obtained by using the tuning fork of same frequency?