Which equation correctly expresses the relationship between speed, frequency, and wavelength for electromagnetic waves in free space?

Which equation correctly expresses the relationship between speed, frequency, and wavelength for electromagnetic waves in free space?

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

Answer: (B)

The speed of a wave is tied directly to how often its cycles occur and how long each cycle is. This relationship is that speed equals frequency times wavelength. For electromagnetic waves in free space, that speed is the speed of light, c, so the equation becomes c = f λ. This is why the correct form is c = f λ: it cleanly links the three quantities with consistent units—c in meters per second, f in hertz (cycles per second), and λ in meters. From this, you can also rearrange to f = c/λ or λ = c/f. The other forms don’t fit: f = c × λ would give wrong units (meters squared per second); v = f + λ mixes quantities that aren’t additive; λ = f / c would yield units of 1/m, not meters.

The speed of a wave is tied directly to how often its cycles occur and how long each cycle is. This relationship is that speed equals frequency times wavelength. For electromagnetic waves in free space, that speed is the speed of light, c, so the equation becomes c = f λ. This is why the correct form is c = f λ: it cleanly links the three quantities with consistent units—c in meters per second, f in hertz (cycles per second), and λ in meters. From this, you can also rearrange to f = c/λ or λ = c/f.

The other forms don’t fit: f = c × λ would give wrong units (meters squared per second); v = f + λ mixes quantities that aren’t additive; λ = f / c would yield units of 1/m, not meters.