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ExploreEnergy and forces

The world, a little closer

Electromagnetic spectrum.

Radio, microwaves, light, X-rays: one kind of wave at different lengths. Slide across fifteen powers of ten and see what changes, and what does not.

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01 / The experiment

One wave, fifteen powers of ten.

Slide the wavelength from radio to gamma. Only a thin slice in the middle is visible light.

Interactive scene

Preparing your experiment…

The explanation below is available while the scene loads.

The on-screen wavelength is compressed onto a log scale; a real radio wave would be a kilometre long and a gamma ray smaller than an atom.Inside the visible band the wave takes the colour of its wavelength. Outside it, the muted colour is only a label; nothing there can be seen.

02 / Three regions

Long, visible, short.

Same speed, same nature. Only the wavelength differs.

Radio to infrared01

Long waves

Photons too gentle to disturb an atom. Radio carries signals through buildings; microwaves twist water molecules; infrared is felt as heat.

Wavelength
Kilometres to a hair
Photon energy
Low
It
Passes walls; warms
400 to 700 nanometres02

Visible light

A sliver of the spectrum, less than one power of ten wide, where the Sun is brightest and the air is clear. Red is long, violet short.

Wavelength
Less than a thousandth of a millimetre
Photon energy
Middling
It
Colour, sight, photosynthesis
Ultraviolet to gamma03

Short waves

Photons energetic enough to damage molecules. Ultraviolet burns skin, X-rays image bones, gamma rays come from nuclei and exploding stars.

Wavelength
Smaller than an atom
Photon energy
High
It
Breaks bonds; passes flesh
The big idea

Every band is the same wave at a different wavelength. Shorter means higher frequency and more energy per photon, and that decides what it can do.

c = f·λ

The same thing at every length

Light, radio, X-rays, the warmth you feel from a fire: all of them are the same kind of thing, a travelling ripple in electric and magnetic fields. They all move at the same speed in empty space, the speed of light, and they differ in exactly one respect: their wavelength, the distance from one crest to the next. Set out from longest to shortest, they form the electromagnetic spectrum.

The range is enormous. Radio waves can be kilometres long; gamma rays are smaller than an atomic nucleus. That is more than fifteen powers of ten, and visible light occupies less than one of them.

Long waves, low energy

Because every wave moves at the same speed, a longer wavelength means fewer crests pass each second: a lower frequency. And frequency is energy. Each packet of a wave, a photon, carries energy in proportion to its frequency. A radio photon carries so little that it can pass through your body without disturbing a single molecule. Slide toward the short end and each photon carries more, until at ultraviolet it can break chemical bonds, and at X-ray it can knock electrons clean out of atoms.

Try it: press Radio, then X-ray. Notice the wave gets shorter and faster, and nothing else about it changes.

The slice we can see

Somewhere near the middle, between about 400 and 700 nanometres, sits the band our eyes evolved to catch: visible light. Red is the long end, violet the short, and every colour of the rainbow is a wavelength in between. Slide through the visible band and the wave takes the colour of that wavelength.

It is no accident that we see this band. The Sun pours out more energy here than anywhere else, and water and air are transparent to it. Other animals see a little beyond: bees into the ultraviolet, some snakes into the infrared.

What passes and what stops

Whether a wave passes through a material depends on how its wavelength matches the material’s structure. A wall’s atoms are far too small for a metre-long radio wave to notice, so the wave sails through; light, a million times shorter, is stopped by the same wall. Microwaves are the right size to twist water molecules, which is why an oven heats food and not the plate. X-rays are so short and energetic that only dense atoms like calcium stop them, which is why a radiograph shows bones against a grey of flesh.

Two fields, one wave

Reveal Both fields and a second ripple appears at right angles to the first. The fine lines from the axis to each curve are field arrows: how strong the field is at that point, and which way it pushes. An electromagnetic wave is an electric field and a magnetic field, each creating the other as they travel, locked in step and perpendicular. Neither needs a medium. That is why sunlight crosses a hundred and fifty million kilometres of nothing to reach us, when sound could not cross a single metre of it.