The phenomenon in which the frequency of the applied force is equal to the natural frequency of the system is known as __________.

CONCEPT:

• Resonance: The phenomenon in which the frequency of the applied force is equal to the natural frequency of the system is known as resonance.
• Scattering: In scattering, when a light wave experiences an obstacle of the order of its wavelength, it deviates from its original trajectory in all directions. Hence, it is a wave phenomenon.
• Interference: When two light waves are incident at the same point, the overall amplitude of the wave at the point is the sum of the individual amplitudes by the superposition principle. This is the cause of interference and hence, it is a wave phenomenon.
• Beat: The combination of two waves of slightly different frequencies, perceived as a periodic variation whose rate is the difference of the two frequencies.

Beat frequency (b) = |f1 - f2|

Where f1 and f2 are two frequencies.

EXPLANATION:

• Resonance: A phenomenon in which amplitude is increased when the frequency of an applied force is equal to the natural frequency of the system on which it acts is known as resonance. 
• Resonance occurs with all types of waves and vibrations.
• Hence the correct answer is option 1.