A certain wave has a speed of 20$ms^{-1}$. If the frequency of the wave is 0.25Hz, calculate the distance between successive crests of the wave

Analyze the question: The distance between successive crests is the wavelength (λ). We are given the wave speed and the frequency.

Concept: The wave equation is

v = fλ, so λ = v / f

Solution:

• v = 20 m/s
• f = 0.25 Hz

λ = 20 / 0.25 = 80 m

Answer: The wavelength is 80 m, which is option D.

Check: A frequency of 0.25 Hz means the period is T = 1/0.25 = 4 s. In 4 seconds the wave travels 20 × 4 = 80 m, which is exactly one wavelength. ✔

Tip: Dividing by 0.25 is the same as multiplying by 4, so 20 ÷ 0.25 = 20 × 4 = 80. Also remember that "distance between successive crests" always means wavelength. The same applies to "distance between successive troughs".