The Electromagnetic Spectrum

This lesson covers the electromagnetic (EM) spectrum — the order of EM waves, their shared properties, and the relationship between frequency and wavelength, as required by the Edexcel GCSE Combined Science specification (1SC0).

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What Are Electromagnetic Waves?

Electromagnetic (EM) waves are transverse waves that transfer energy from one place to another. Unlike sound, they do not need a medium — they can travel through a vacuum.

Key Properties of All EM Waves

Property	Value / Detail
Type	Transverse
Speed in a vacuum	3.0 × 10⁸ m/s (the speed of light, c)
Do they need a medium?	No — they can travel through a vacuum
Made of	Oscillating electric and magnetic fields at right angles to each other

Exam Tip: All EM waves travel at the same speed in a vacuum: 3.0 × 10⁸ m/s. This is given on the data sheet but you should memorise it to save time.

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The EM Spectrum — Order

The EM spectrum is a continuous range of waves, arranged by wavelength and frequency.

graph LR
    A["Radio waves"] --> B["Microwaves"]
    B --> C["Infrared"]
    C --> D["Visible light"]
    D --> E["Ultraviolet"]
    E --> F["X-rays"]
    F --> G["Gamma rays"]
    style A fill:#ff9999
    style B fill:#ffcc99
    style C fill:#ffff99
    style D fill:#99ff99
    style E fill:#99ccff
    style F fill:#cc99ff
    style G fill:#ff99ff

From longest wavelength / lowest frequency to shortest wavelength / highest frequency:

EM Wave	Approximate Wavelength Range	Approximate Frequency Range
Radio waves	> 1 m (up to km)	< 3 × 10⁸ Hz
Microwaves	1 mm – 1 m	3 × 10⁸ – 3 × 10¹¹ Hz
Infrared (IR)	700 nm – 1 mm	3 × 10¹¹ – 4.3 × 10¹⁴ Hz
Visible light	400 nm – 700 nm	4.3 × 10¹⁴ – 7.5 × 10¹⁴ Hz
Ultraviolet (UV)	10 nm – 400 nm	7.5 × 10¹⁴ – 3 × 10¹⁶ Hz
X-rays	0.01 nm – 10 nm	3 × 10¹⁶ – 3 × 10¹⁹ Hz
Gamma rays	< 0.01 nm	> 3 × 10¹⁹ Hz

Memory Aid

A popular mnemonic: Runny Mashed In Very Ugly Xmas Gravy

(Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma)

Exam Tip: You MUST be able to list the EM spectrum in order. This appears in nearly every exam series. Learn the mnemonic.

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The Relationship Between Frequency and Wavelength

For all EM waves in a vacuum:

$c = f \lambda$c=fλ

where c = 3.0 × 10⁸ m/s.

Since the speed is constant:

• Higher frequency → shorter wavelength
• Lower frequency → longer wavelength

Direction	Frequency	Wavelength
Radio → Gamma	Increases	Decreases
Gamma → Radio	Decreases	Increases

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Visible Light

Visible light is the only part of the EM spectrum that the human eye can detect. It has wavelengths from approximately 400 nm (violet) to 700 nm (red).

Colour	Approximate Wavelength
Violet	400 nm
Blue	450 nm
Green	520 nm
Yellow	570 nm
Orange	600 nm
Red	700 nm

• White light is a mixture of all the colours of the visible spectrum.
• A prism can split white light into its component colours by dispersion (different wavelengths refract by different amounts).

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EM Waves and Energy

The energy carried by an EM wave is related to its frequency:

• Higher frequency = higher energy
• Lower frequency = lower energy

This means:

• Gamma rays carry the most energy.
• Radio waves carry the least energy.
• The higher the energy, the more dangerous the wave can be to living tissue.

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Worked Example 1

A radio wave has a wavelength of 1500 m. Calculate its frequency.

$f = \frac{c}{\lambda} = \frac{3.0 \times 10^{8}}{1500} = 2.0 \times 10^{5} \text{ Hz} = 200 \text{ kHz}$f=λc​=15003.0×108​=2.0×105 Hz=200 kHz

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Worked Example 2

An X-ray has a frequency of 6.0 × 10¹⁷ Hz. Calculate its wavelength.

$\lambda = \frac{c}{f} = \frac{3.0 \times 10^{8}}{6.0 \times 10^{17}} = 5.0 \times 10^{-10} \text{ m} = 0.5 \text{ nm}$λ=fc​=6.0×10173.0×108​=5.0×10−10 m=0.5 nm

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Worked Example 3

Green light has a wavelength of 5.2 × 10⁻⁷ m. Calculate its frequency.

$f = \frac{c}{\lambda} = \frac{3.0 \times 10^{8}}{5.2 \times 10^{-7}} = 5.77 \times 10^{14} \text{ Hz (3 s.f.)}$f=λc​=5.2×10−73.0×108​=5.77×1014 Hz (3 s.f.)

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How EM Waves Are Produced and Detected

EM Wave	How Produced	How Detected
Radio waves	Oscillating electrons in a transmitter circuit	Aerial / antenna
Microwaves	Magnetron in microwave oven, transmitters	Aerial / receiver
Infrared	Warm and hot objects	Skin (warmth), infrared camera, thermometer
Visible light	Very hot objects, LEDs, lasers	Eye, photographic film, LDR
Ultraviolet	Very hot objects (e.g. the Sun), UV lamps	Fluorescent materials, UV camera
X-rays	X-ray tube (high-energy electrons hitting a metal target)	Photographic film, electronic detector
Gamma rays	Radioactive decay of unstable nuclei	Geiger–Müller tube, photographic film

Exam Tip: You could be asked how a specific type of EM wave is produced or detected. Make sure you know at least one method for each.

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Summary

• EM waves are transverse waves that can travel through a vacuum at 3.0 × 10⁸ m/s.
• The spectrum (in order of increasing frequency): Radio, Microwave, Infrared, Visible, UV, X-ray, Gamma.
• All EM waves obey $c = f \lambda$c=fλ — higher frequency means shorter wavelength.
• Visible light ranges from about 400 nm (violet) to 700 nm (red).
• Higher-frequency EM waves carry more energy and are potentially more dangerous.
• EM waves are produced and detected in different ways depending on their type.

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Why the EM Spectrum Is Continuous

Radio waves, microwaves, infrared and the rest are not separate kinds of wave — they are all the same physical phenomenon: an oscillating electric field coupled to an oscillating magnetic field, travelling through space at the speed of light. The only difference between them is their frequency (and therefore their wavelength and energy). The boundaries between "microwaves" and "infrared", or between "UV" and "X-rays", are chosen historically and by application, not by any change in the physics.