In free space, which equation correctly relates propagation speed v, frequency f, and wavelength λ?

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

Answer: (A)

The speed of a wave is given by multiplying its frequency by its wavelength, so v = f λ. This makes sense because frequency tells how many wave cycles pass each second and wavelength tells how far a single cycle travels; together they determine how far the wavefront moves per second. In free space, that speed is the speed of light, so you can also say λ = c / f if you know the frequency. The other options don’t match the units or the fundamental relation: f/λ has units of per meter, not speed; λ/f has units of meters·seconds, not speed; and c f would have units of meters per second squared, not a speed.