One phenomenon, many faces
Radio waves, the warmth from a heater, the light you are reading by, and the gamma rays from a distant star are all the same thing: electromagnetic radiation. They differ only in wavelength (λ) — and everything else follows from that. The interactive above is one continuous bar from the longest radio waves on the left to the shortest gamma rays on the right. Slide the marker (or focus it and use the arrow keys) and watch three numbers move in lockstep.
How the three numbers are linked
All electromagnetic waves travel through a vacuum at the speed of light, c ≈ 3.0 × 10⁸ m/s. That single fact ties wavelength and frequency together:
- f = c / λ — shorter wavelengths mean higher frequencies.
- E = h · f — a photon’s energy is its frequency times Planck’s constant, h ≈ 4.14 × 10⁻¹⁵ eV·s.
So as you slide to the right, λ shrinks, f climbs, and the energy carried by each photon rises. That is why gamma rays are dangerous and radio waves are not: a single gamma photon carries millions of times the energy of a radio photon. The widget formats each value with sensible units — km, m, cm, mm, µm, nm, pm for wavelength; Hz up to THz for frequency; meV up to MeV for photon energy.
The visible sliver
Notice how narrow the visible band is — roughly 400 nm (violet) to 700 nm (red), a tiny slice of the whole spectrum. Inside it the bar becomes a rainbow, and the readout shows the approximate colour for the wavelength you have landed on. Green sits near 550 nm, with a photon energy of about 2.26 eV. Everything to the left of red is too low-energy for our eyes (infrared, microwave, radio); everything past violet is too high-energy (ultraviolet, X-ray, gamma).
Using this with a class
Project it and ask students to predict, before you slide: “If we double the frequency, what happens to the wavelength and the photon energy?” Or name a technology — a microwave oven, a chest X-ray, an FM station — and have a student move the marker to the right region and read off the numbers. It is free to embed on your own class site or LMS.