Electromagnetic radiation

Electromagnetic radiation (light, radio waves, X-rays and so on) consists of oscillating electric and magnetic fields, at right angles to each other and to the direction of travel. Unlike sound, it needs no medium: it can travel through empty space.

Three quantities describe a wave:

• Wavelength (λ): the distance between two neighbouring crests.
• Frequency (ν): the number of waves passing a point per second, in hertz (Hz, s⁻¹).
• Wavenumber (ν̄): the number of wavelengths per unit length, ν̄ = 1/λ, in m⁻¹ or, commonly, cm⁻¹.

All electromagnetic waves travel in a vacuum at the same speed, c = 3.0 × 10⁸ m s⁻¹, and c = νλ. So a shorter wavelength means a higher frequency. For example, green light of wavelength 500 nm has frequency ν = c/λ = 3.0 × 10⁸ ÷ 500 × 10⁻⁹ = 6.0 × 10¹⁴ Hz.

The electromagnetic spectrum, in order of increasing frequency (decreasing wavelength): radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and cosmic rays. Visible light is only a narrow band, from violet (about 400 nm) to red (about 750 nm).

Thinking of light as a wave explains diffraction (bending round obstacles) and interference. But it could not explain black-body radiation or the photoelectric effect. That needed a new idea.