The graph shows how the vertical height of a travelling wave varies with distance along the path of the wave - AQA - A-Level Physics - Question 13 - 2017 - Paper 1

To find the period of the wave, we can use the relationship between speed, wavelength, and frequency. The formula is:

$v = f \cdot \lambda$

Where:

• $v$ is the wave speed (20 cm/s)
• $f$ is the frequency
• $\lambda$ is the wavelength

From the graph, we observe that the wavelength ($\lambda$) is the distance for one full cycle of the wave. By examining the graph, we see that the distance from one peak to the next is approximately 4 cm, so:

$\lambda = 4 \text{ cm}$

Now, substituting in the wave speed:

$20 = f \cdot 4$

Solving for frequency $f$ gives:

$f = \frac{20}{4} = 5 \text{ Hz}$

Next, we find the period $T$, which is the inverse of frequency:

$T = \frac{1}{f} = \frac{1}{5} = 0.2 \text{ s}$

Therefore, the period of the wave is 0.2 s, which corresponds to option B.