Colour & the Spectrum: Most Light Is Invisible
A prism doesn't add colour to sunlight — it un-mixes what was always there. And the rainbow you can see is one octave of a keyboard that runs from radio to gamma.
Builds on: 5.1 Reflection & Refraction: Light Changes Lanes4.1 Waves: Motion Without Transport
White is a chord
Newton let sunlight through a prism and got a rainbow — then, the decisive step, fed it through a second prism and recombined it into white. Colour is not added by the glass; sunlight is a mixture, and the prism sorts it. The sorting works because a medium’s refractive index depends slightly on wavelength — dispersion: violet slows a touch more than red, so it bends a touch more, and the mixture fans out. Raindrops do the same job, sorted across the sky.
Colour is wavelength: deep red ~700 nm down to violet ~400 nm. Your eye samples this range with three cone types and your brain mixes their votes — which is why a screen fools you with only red, green and blue pixels.
The keyboard beyond
The visible band is a sliver of the electromagnetic spectrum — one physical phenomenon at every wavelength, all doing c:
- Radio (metres–kilometres) — broadcast, Wi-Fi’s 12 cm.
- Microwaves (cm–mm) — radar, and rotating your leftovers’ water molecules.
- Infrared (µm) — heat radiation: night-vision sees warm bodies glow.
- Visible — 400–700 nm. This sliver.
- Ultraviolet — energetic enough to break skin chemistry: sunburn.
- X-rays / gamma — through flesh, out of nuclei.
Why do eyes use this sliver? The Sun’s output peaks here, and — no coincidence — water and air are transparent here. Evolution tuned the receiver to the channel that was open.
Why the sky is blue (and sunsets red)
Air molecules scatter light — weakly, but with a savage wavelength bias: scattering strength goes as 1/λ⁴, so 450 nm blue is bounced around several times more than 650 nm red. Look anywhere but at the Sun and you see this scattered blue haze. At sunset the light crosses so much air that the blue has been scattered out of the beam entirely — what survives the long path is the red, painted on the clouds.
Look closely at a star’s spectrum and dark lines interrupt it — each element in its atmosphere absorbs its own precise wavelengths. Helium was discovered in the Sun’s lines before anyone found it on Earth (hence the name, from *helios*). Those sharp, quantised lines are also a loose thread: pull it and quantum physics unravels out — next unit.
⚗️ Lab — The Prism Bench
White light in, spectrum out — with a wavelength slider to walk the whole keyboard.
- Watch violet bend harder than red through the prism: dispersion, live.
- Drag the wavelength marker out of the visible band in both directions — the labels keep going.
- Check the scattering meter as you go blue: 1/λ⁴ is why the sky is up there.
Work it by hand — 0 / 2
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±2% count.