Two identical tuning forks are placed side by side. Why does the vibration of the first tuning fork cause the vibration of the second?
This is an example of resonance.
Both the tuning forks are given to be identical.
These two tuning forks are connected to each other by the surrounding air. The air particles surrounding the first tuning fork start vibrating due to vibrations of its tines. The pressure waves thus created begin to impinge at a periodic and regular rate upon the second tuning fork. The energy carried by the sound waves through the air is tuned to the frequency of the second tuning fork.
Since the incoming sound waves have the same natural frequency, the second tuning fork begins to vibrate easily at its natural frequency.