What you are looking at
A travelling electromagnetic wave drawn in 3D. The
red curve is the
electric field E, oscillating up and down; the
blue
curve is the
magnetic field B, oscillating into and out of the page. Both are perpendicular
to each other and to the
direction of travel (to the right) — this is a
transverse wave. The whole pattern marches to the right at the speed of light.
Fields that make each other
Maxwell's great discovery was that a changing electric field creates a magnetic field, and a changing
magnetic field creates an electric field. Once started, the two fields keep regenerating each other and the
disturbance propagates on its own — no medium required, which is why light crosses empty space. The two
fields oscillate
in phase (they peak together) and their strengths are linked by E = cB.
The wave travels at a speed fixed entirely by the electric and magnetic properties of the vacuum:
c = 1 / √(μ₀ ε₀) ≈ 3.00 × 10⁸ m/s
Wavelength, frequency and energy
For every electromagnetic wave the speed is the same, so wavelength and frequency trade off:
c = λ f E_photon = h f = h c / λ
A long wavelength means a low frequency and low-energy photons; a short wavelength means high frequency and
high-energy photons. Slide across the spectrum and watch the wave bunch up (shorter λ, more cycles, faster
oscillation) while the speed stays pinned at c.
The electromagnetic spectrum
Radio, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays are all the same kind of
wave — they differ only in wavelength, spanning more than twenty orders of magnitude. Our eyes respond to a
tiny slice (about 400–700 nm). The high-energy end (UV, X-ray, gamma) carries enough energy per photon to
damage molecules, while the low-energy end (radio, microwave) is what we use to broadcast and communicate.
Things to try
Jump between bands with the buttons, or sweep the wavelength slider to morph continuously from kilometre-long
radio waves to picometre gamma rays, watching the frequency and photon energy change by huge factors while c
never budges. Toggle the E and B fields on and off to see them separately, and turn the field vectors on to
see the arrows at each point along the wave.