The Electromagnetic Spectrum

Visible light — the rainbow of colours we can see — is only a tiny slice of a much larger family of waves. Radio waves carrying your favourite station, the microwaves that heat your dinner, the infrared warmth of the Sun, the ultraviolet that tans (and burns) your skin, the X-rays that image a broken bone, and the gamma rays used to treat cancer are all the same kind of wave as visible light. Together they make up the electromagnetic spectrum: a continuous family of transverse waves that all travel at the same enormous speed through a vacuum, differing only in their frequency and wavelength. This lesson, part of Topic P5 (Waves in matter) of OCR Gateway Science A, sets out the seven groups of the spectrum in order, the properties they share, and the trends in frequency, wavelength and energy across it.

By the end of this lesson you should be able to name the seven groups of the electromagnetic spectrum in order, state the properties all electromagnetic waves share, describe how frequency, wavelength and energy change across the spectrum, and explain that the spectrum is continuous.

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What Are Electromagnetic Waves?

Electromagnetic (EM) waves are transverse waves that transfer energy from a source to an absorber. Unlike sound, they do not need a medium — they can travel through a vacuum, which is how light and radio waves reach us across the emptiness of space from the Sun and from satellites.

All electromagnetic waves share a set of properties:

• they are all transverse;
• they all travel at the same speed in a vacuum — the speed of light, $3 \times 10^8 \text{ m/s}$3×108 m/s (300 million metres per second);
• they all transfer energy from a source to an absorber;
• they can all travel through a vacuum (empty space).

What makes one type of EM wave different from another is its frequency and wavelength. Because they all travel at the same speed $v = 3 \times 10^8 \text{ m/s}$v=3×108 m/s, the wave equation $v = f\lambda$v=fλ tells us that a higher frequency must go with a shorter wavelength, and vice versa.

Exam Tip: The properties all EM waves share are worth memorising: transverse, transfer energy, travel at $3 \times 10^8 \text{ m/s}$3×108 m/s in a vacuum, and can cross a vacuum. A question asking "what do all EM waves have in common?" is testing exactly these.

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The Seven Groups of the Spectrum

The electromagnetic spectrum is divided into seven groups. In order of increasing frequency (and therefore decreasing wavelength) they are:

radio → microwave → infrared → visible light → ultraviolet → X-rays → gamma rays

A common mnemonic to remember the order is "Rich Men In Vegas Use eXpensive Gamblers" — Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma.

• Radio waves have the longest wavelength and lowest frequency (and lowest energy);
• Gamma rays have the shortest wavelength and highest frequency (and highest energy);
• Visible light sits in the middle and is the only part our eyes can detect.

The table below sets out the spectrum in order, with the trends in wavelength, frequency and energy.

Group (in order)	Wavelength	Frequency	Energy
Radio waves	Longest	Lowest	Lowest
Microwaves	↓	↑	↑
Infrared	↓	↑	↑
Visible light	↓	↑	↑
Ultraviolet	↓	↑	↑
X-rays	↓	↑	↑
Gamma rays	Shortest	Highest	Highest

Reading down the table (from radio to gamma): the wavelength decreases, while the frequency and the energy of the waves increase. Reading up the table reverses all three trends.

Exam Tip: Learn the order radio → micro → infrared → visible → ultraviolet → X-ray → gamma and that frequency increases (wavelength decreases) from radio to gamma. Whether you go up or down, frequency and wavelength always change in opposite directions, because $v$v is fixed and $v = f\lambda$v=fλ.

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Visualising the Spectrum

The diagram below shows the spectrum as a band, with the long-wavelength, low-frequency radio waves at one end and the short-wavelength, high-frequency gamma rays at the other. Visible light is the narrow strip in the middle that our eyes can see.

The spectrum is also a tiny slice expressed in numbers: radio waves can have wavelengths of kilometres, while gamma rays have wavelengths smaller than an atom. Visible light's wavelength is around a millionth of a metre — within visible light itself, red has the longest wavelength (lowest frequency) and violet the shortest (highest frequency), which is why a prism spreads white light into the familiar rainbow from red to violet. The full sequence of colours within visible light, from longest to shortest wavelength, is red, orange, yellow, green, blue, indigo, violet — so red light has a lower frequency than blue light, even though both are "visible".

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How Each Part of the Spectrum Is Made and Detected

Different parts of the spectrum come from different sources and need different detectors, even though they are all the same kind of wave:

• Radio waves are produced by oscillating charges (an alternating current) in an aerial, and detected by an aerial in which the waves induce a current.
• Infrared is emitted by hot objects — anything warm gives out infrared — and detected by the skin (felt as heat), by special cameras, or by infrared sensors.
• Visible light comes from very hot objects (the Sun, a lamp filament, a flame) and is detected by the eye or by a camera sensor.
• Ultraviolet, X-rays and gamma rays come from very high-energy sources: UV from the Sun and special lamps, X-rays from X-ray tubes, and gamma rays from the nuclei of radioactive atoms. They are detected by photographic film or electronic detectors.

The general rule is that the higher-energy waves come from higher-energy processes — radio waves from gentle oscillations of charge, but gamma rays from changes inside the atomic nucleus.