Speed, frequency and wavelength

A travelling wave

Light is an electromagnetic wave: it corresponds to the propagation through space of an oscillating electric field and a magnetic field, without the transport of matter. In a vacuum, it travels at the speed c = 3.00 × 10⁸ m/s, a record value: nothing travels any faster.

Three related quantities

A light wave is characterised by:

• its period T (in seconds): the duration of one complete oscillation
• its frequency f = 1/T (in hertz, Hz): the number of oscillations per second
• its wavelength λ (in metres): the distance travelled by the wave during one period

These quantities are related by the fundamental equation:

lambda = c / f, or c = lambda × f

Quantity	Symbol	Unit
Wavelength	lambda	m
Frequency	f	Hz
Speed of light	c	m/s

Example

Red light with a wavelength lambda = 700 nm = 700 × 10⁻⁹ m has a frequency: f = c / lambda = (3.00 × 10⁸) / (700 × 10⁻⁹) ≈ 4.3 × 10¹⁴ Hz

Common pitfall

Note: when light passes from a vacuum (or air) into another medium (water, glass), its speed and wavelength change, but its frequency remains constant. It is the frequency, not the wavelength, that universally characterises a colour.