What is wavelength?

• A. The energy of a photon
• B. The speed of light
• C. The height of a wave crest
• D. The physical length of one complete cycle

Answer: (D)

Wavelength is the physical length of one complete cycle of a wave—the distance the pattern travels before it repeats. You can think of it as crest to crest (or trough to trough) along the direction the wave moves. This spatial period defines how long the pattern lasts in space and, for light, relates to frequency by λ = c / f, so higher frequency waves have shorter wavelengths. The energy of a photon depends on frequency (E = h f), which ties into wavelength through E = hc/λ, but that energy relation doesn’t define the wavelength itself. The speed of light describes how fast the wave propagates, and the height of a crest is the amplitude, not the wavelength. So the distance of one complete cycle is the wavelength.