A Level Physics Practice Exam

The frequency of harmonics in a standing wave system is determined by the fundamental frequency, with higher harmonics being integer multiples of that fundamental frequency.

For the first harmonic, also known as the fundamental frequency, the frequency is given as 20 Hz. The second harmonic would then have a frequency that is twice the fundamental frequency, which is 40 Hz. The third harmonic, which is the focus of the question, would be three times the fundamental frequency. Therefore, the frequency of the third harmonic can be calculated as follows:

Third Harmonic = 3 × Fundamental Frequency = 3 × 20 Hz = 60 Hz.

This calculation shows that the correct frequency for the third harmonic is 60 Hz.