For a wave, the speed equals the product of its frequency and its wavelength.

• A. Speed = Frequency × Wavelength
• B. Speed = Frequency / Wavelength
• C. Speed = Wavelength / Frequency
• D. Speed is independent of frequency and wavelength

Answer: (A)

The main idea is that a wave’s speed comes from how often crests pass a point and how far apart those crests are. In one second, f crests pass, and each crest is separated by the wavelength λ, so the distance the wave pattern travels in that second is f × λ. That distance per second is the speed, giving v = fλ. This works for all waves, including light, where the speed of light c equals fλ; changing frequency changes the wavelength so their product stays the same.

The option that uses the product f × λ matches this physical relationship and has the correct units: Hz (or s⁻¹) times meters gives meters per second.

Dividing frequency by wavelength would yield units of 1/(s·m), not speed. Wavelength divided by frequency would give meters·seconds, which also isn’t speed. Saying speed is independent of frequency and wavelength misses the direct link between how often the pattern repeats and how far apart the repeats are.