The electromagnetic spectrum

“Visible light” is only a tiny part of the spectrum of electromagnetic waves. As frequency and wavelength vary, very different phenomena occur, but all are governed by the same Maxwell equations and propagate at the same speed $c$ in vacuum.

Type	Typical $\lambda$	Example uses
Radio waves	$> 1$ m	broadcasting, Wi-Fi
Microwaves	1 mm – 1 m	radar, microwave oven
Infrared	700 nm – 1 mm	night vision, remote controls
Visible	400–700 nm	human eyes
Ultraviolet	10–400 nm	tanning, sterilisation
X-rays	0.01–10 nm	radiography
$\gamma$ rays	$< 0{,}01$ nm	radioactivity, astrophysics

Since $\lambda\,f = c$, a small wavelength corresponds to a high frequency and — as we shall see with photons — high energy: $\gamma$ rays are the most energetic, radio waves the least energetic.

The visible is a small window

Visible waves have wavelengths between $\sim 400$ nm (violet) and $\sim 700$ nm (red), corresponding to photons of energy $\sim 2$–$3$ eV: precisely the right energy to excite rhodopsin in our eyes. This is no coincidence: these values coincide with the peak of solar irradiance. Evolution has “tuned” human sight to the most abundant band of the spectrum.

Links

Topics: Electromagnetic waves Concepts: Electromagnetic wave · Photon

Related exercises: Problem — Microwave oven · Problem — Which EM wave · Problem — FM wavelength (100 MHz)