Electromagnetic Spectrum

Of all the wave phenomena in physics, perhaps the most astonishing is that visible light — the radiation of familiar daylight — is only a very narrow slice of a much larger family of waves which all share the same fundamental properties. From the kilometre-long radio waves carrying radio broadcasts to the picometre-short gamma rays released from nuclear reactions, the electromagnetic spectrum is a single continuous family of transverse waves all travelling at the same speed in vacuum — the speed of light, c = 3.00 × 10⁸ m s⁻¹.

This lesson summarises the regions of the spectrum, their properties and some of their uses, and develops the central physical fact that in vacuum all electromagnetic waves have the same speed regardless of frequency.

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What Is Electromagnetic Radiation?

In the nineteenth century, James Clerk Maxwell showed mathematically that oscillating electric and magnetic fields could self-propagate through empty space at a speed c given entirely by two electrical constants — the permittivity and permeability of free space:

c = 1 / √(ε₀μ₀) ≈ 3.00 × 10⁸ m s⁻¹

This result was one of the great triumphs of nineteenth-century physics. Maxwell realised that his theoretical prediction matched the measured speed of light (determined in the 1850s by Fizeau and Foucault) and concluded — boldly — that light is an electromagnetic wave.

All electromagnetic waves consist of mutually perpendicular oscillating electric (E) and magnetic (B) fields, each perpendicular to the direction of propagation. They are therefore transverse waves. They need no medium: they can travel through empty space, which is why light from distant stars can reach us across billions of light-years of vacuum.

flowchart LR
    subgraph Fields["Oscillating field picture"]
        E[Electric field E, vertical]
        B[Magnetic field B, horizontal]
        P[Propagation direction]
    end
    E -- perpendicular --> B
    E -- perpendicular --> P
    B -- perpendicular --> P
    P --> V[Speed in vacuum: c = 3 x 10^8 m/s]

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Key Facts About Electromagnetic Waves

All electromagnetic waves share the following properties. These are frequently tested.

1. They are transverse. (Both E and B oscillate perpendicular to the direction of propagation.)
2. They travel at c = 3.00 × 10⁸ m s⁻¹ in vacuum, regardless of frequency.
3. They can be polarised. (Since they are transverse — see Lesson 4.)
4. They do not require a medium — they can travel through vacuum.
5. They obey the wave equation v = fλ.
6. They can be reflected, refracted, diffracted and interfered — all the standard wave behaviours.
7. Their speed depends on the medium. In glass, for example, visible light slows to about 2 × 10⁸ m s⁻¹, giving a refractive index of ~1.5.
8. Their frequency is unchanged on refraction. Only speed and wavelength change.

Exam Tip: The value c = 3.00 × 10⁸ m s⁻¹ is given in the OCR data, formulae and relationships booklet, but you should still know it by heart. Expect calculations to use this rounded value throughout the exam.

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The Regions of the Spectrum

The full electromagnetic spectrum, in order of increasing frequency (and decreasing wavelength), is:

Radio → Microwave → Infrared → Visible → Ultraviolet → X-rays → Gamma rays

These boundaries are not sharp; they reflect the historical circumstances in which each region was discovered and are also influenced by the physical mechanism by which each type of wave is typically produced or detected. The table below summarises the typical wavelengths, frequencies and applications.

Region	Wavelength (m)	Frequency (Hz)	Typical source	Typical use
Radio	> 10⁻¹ (m to km)	< 3 × 10⁹	Oscillating electrons in aerials	Broadcasting, communications
Microwave	10⁻³ to 10⁻¹	3 × 10⁹ to 3 × 10¹¹	Magnetrons, klystrons	Satellite links, radar, cooking
Infrared (IR)	7 × 10⁻⁷ to 10⁻³	3 × 10¹¹ to 4 × 10¹⁴	Hot bodies	Thermal imaging, TV remote controls
Visible	4 × 10⁻⁷ to 7 × 10⁻⁷	4 × 10¹⁴ to 7.5 × 10¹⁴	The Sun, filament lamps, LEDs	Vision, photography
Ultraviolet (UV)	10⁻⁸ to 4 × 10⁻⁷	7.5 × 10¹⁴ to 3 × 10¹⁶	Very hot bodies, UV lamps	Fluorescence, sterilisation, sun-tanning
X-rays	10⁻¹² to 10⁻⁸	3 × 10¹⁶ to 3 × 10²⁰	Electron deceleration in X-ray tubes	Medical imaging, crystallography
Gamma rays	< 10⁻¹²	> 3 × 10¹⁹	Nuclear decay	Sterilisation, cancer therapy

You are expected to know this order and to have a rough idea of wavelengths and frequencies in each region.