What are ordering the regions?

From lowest frequency (longest wavelength) to highest frequency (shortest wavelength) the regions are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Visible light spans roughly 400\ nm (violet) to 700\ nm (red). Each region is produced and detected differently: radio waves by oscillating circuits, infrared by warm objects, X-rays by rapidly decelerating electrons, and gamma rays by nuclear processes.

WAPhysicsSyllabus dot point

How are the regions of the electromagnetic spectrum related and produced?

Describe the electromagnetic spectrum and relate frequency, wavelength and the speed of light

A focused answer to the WACE Year 12 Physics Unit 4 content point on the electromagnetic spectrum. The common nature of all EM waves, the wave equation linking frequency and wavelength, the ordering of the regions, and how photon energy varies across the spectrum.

Generated by Claude Opus 4.76 min answerUpdated 2026-05-29

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking

WACE wants you to describe the electromagnetic spectrum as a single family of waves, apply the wave equation, and order the regions by frequency and wavelength. The unifying idea is that visible light is just one narrow band of a much larger continuous spectrum.

One family of waves

Every electromagnetic wave is a self-propagating disturbance of electric and magnetic fields oscillating at right angles to each other and to the direction of travel. They need no medium and all travel at the same speed $c$ in a vacuum. What distinguishes radio from gamma rays is only their frequency and wavelength, not their fundamental nature.

The wave equation

For any electromagnetic wave,

c=f\lambda,

so frequency and wavelength are inversely related at fixed speed. A higher frequency means a shorter wavelength. This single relationship lets you convert between the two anywhere in the spectrum.

Ordering the regions

From lowest frequency (longest wavelength) to highest frequency (shortest wavelength) the regions are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Visible light spans roughly $400\ \text{nm}$ (violet) to $700\ \text{nm}$ (red). Each region is produced and detected differently: radio waves by oscillating circuits, infrared by warm objects, X-rays by rapidly decelerating electrons, and gamma rays by nuclear processes.

Photon energy across the spectrum

Treating light as photons, each carries energy

E=hf=\frac{hc}{\lambda}.

So the same ordering by frequency is an ordering by photon energy: radio photons are very low energy, while X-ray and gamma photons are highly energetic and ionising. This is why gamma rays and X-rays are penetrating and biologically damaging, while radio and microwave photons are not ionising.

Frequency and photon energy of an X-ray

From wavelength to frequency to energy

An X-ray has a wavelength of $1.0\times10^{-10}\ \text{m}$ . Find its frequency and the energy of one photon ( $h=6.63\times10^{-34}\ \text{J s}$ ).

Frequency from $c=f\lambda$ :

f=\frac{c}{\lambda}=\frac{3.0\times10^8}{1.0\times10^{-10}}=3.0\times10^{18}\ \text{Hz}.

Photon energy:

E=hf=(6.63\times10^{-34})(3.0\times10^{18})=2.0\times10^{-15}\ \text{J}.

That is about $12000\ \text{eV}$ , far more than a visible photon (a few eV), which is why X-rays are ionising.

Linking the relationships

Use $c=f\lambda$ to move between frequency and wavelength, and $E=hf$ to find photon energy. Keep wavelengths in metres and watch the powers of ten, since the spectrum spans more than fifteen orders of magnitude in frequency.

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