Which equation correctly relates wave speed v, frequency f, and wavelength λ?

Which equation correctly relates wave speed v, frequency f, and wavelength λ?

• A. v = f λ
• B. v = λ/ f
• C. v = f/λ
• D. v = c λ

Answer: (A)

The main concept is that a wave’s speed is the distance the wave travels each second. A single cycle spans one wavelength, so the distance traveled per cycle is λ. If the wave makes f cycles every second, the total distance covered per second is f times λ. That gives v = fλ. This works for all waves, including light in vacuum where fλ equals the speed of light. The other forms would misplace factors or mix in a quantity with incompatible units (for example, λ/ f would have wrong units, and cλ would give units of m^2/s), so they don’t describe speed correctly.

The main concept is that a wave’s speed is the distance the wave travels each second. A single cycle spans one wavelength, so the distance traveled per cycle is λ. If the wave makes f cycles every second, the total distance covered per second is f times λ. That gives v = fλ. This works for all waves, including light in vacuum where fλ equals the speed of light. The other forms would misplace factors or mix in a quantity with incompatible units (for example, λ/ f would have wrong units, and cλ would give units of m^2/s), so they don’t describe speed correctly.