The Electromagnetic Spectrum: KS3 Physics Guide

The electromagnetic spectrum is the complete family of transverse waves that all travel at the speed of light — 300,000,000 m/s — through a vacuum. Sorted by increasing frequency and decreasing wavelength, it runs from radio waves through to gamma rays, with visible light occupying only a tiny band in the middle.

What properties do all electromagnetic waves share?

Before we explore the seven regions, it is worth asking yourself: what do radio waves and gamma rays actually have in common? Quite a lot, it turns out.

All electromagnetic waves are transverse waves — the oscillations are perpendicular to the direction the wave travels. Imagine shaking a rope up and down while it moves horizontally; that is the geometry. All seven types travel at exactly 3 × 10⁸ m/s (300,000,000 m/s) through a vacuum — the speed of light. They transfer energy from place to place without transferring any matter.

They are also connected by the wave equation v = f × λ, where v is wave speed, f is frequency, and λ (lambda) is wavelength. Because v is constant for all electromagnetic waves in a vacuum, increasing frequency must mean decreasing wavelength, and vice versa. Unlike sound, which is a mechanical wave requiring particles to vibrate, electromagnetic waves are self-sustaining oscillations of electric and magnetic fields — they need no medium and travel happily through empty space. The key differences between the seven regions come down to frequency, wavelength, and energy per photon.

What are the seven regions of the electromagnetic spectrum?

What do you think will happen to wavelength as you move from radio waves towards gamma rays? Predict first, then check against the table below. The spectrum is continuous — there are no sharp edges between regions — but the table gives you the accepted approximate values.

Region | Approximate wavelength | Approximate frequency | Example sources
Radio waves | >0.1 m | <3 × 10⁹ Hz | Radio transmitters, stars
Microwaves | 1 mm – 0.1 m | 3 × 10⁹ – 3 × 10¹¹ Hz | Microwave ovens, Wi-Fi, satellites
Infrared | 700 nm – 1 mm | 3 × 10¹¹ – 4 × 10¹⁴ Hz | Hot objects, remote controls, thermal cameras
Visible light | 400 – 700 nm | 4 × 10¹⁴ – 7 × 10¹⁴ Hz | The Sun, lamps, LEDs
Ultraviolet | 10 – 400 nm | 7 × 10¹⁴ – 3 × 10¹⁶ Hz | The Sun, UV lamps, black lights
X-rays | 0.01 – 10 nm | 3 × 10¹⁶ – 3 × 10¹⁹ Hz | X-ray tubes in hospitals
Gamma rays | <0.01 nm | >3 × 10¹⁹ Hz | Radioactive nuclei, supernovae

Visible light spans the colours of the rainbow — ROYGBIV (red through to violet). Red light has the lowest frequency within the visible band; violet has the highest. The Sun is the most familiar source of the full visible spectrum, and white light is a mixture of all colours.

What are the main uses of each type of electromagnetic radiation?

Think about the television signal reaching your aerial, the food warming in your kitchen, and the X-ray taken at a hospital. All of these rely on different parts of the spectrum.

Radio waves carry TV and radio broadcasts, mobile phone signals, radar pulses, and are used by radio telescopes to study the universe. Their long wavelengths allow them to diffract around buildings and hills, which is why you can often receive a signal even without line-of-sight to the transmitter.

Microwaves are absorbed efficiently by water molecules, making them ideal for cooking — the water molecules vibrate and heat the food. They also carry satellite communications and mobile phone network signals over long distances.

Infrared radiation is emitted by all warm objects. TV remote controls send coded pulses of infrared to the receiver. Thermal imaging cameras used by firefighters detect the infrared emitted by hot spots behind walls. Infrared is also used in optical fibre communications, carrying data as pulses of light.

Visible light is essential for human vision, photography, lasers for cutting and scanning, and photosynthesis in plants. Optical fibres for the internet also carry visible light or near-infrared pulses.

Ultraviolet light is used to sterilise medical equipment and drinking water (UV damages the DNA of bacteria, killing them), to detect forged banknotes (security features fluoresce under UV), and it triggers vitamin D production in human skin.

X-rays pass through soft tissue but are absorbed by denser bone, making them invaluable for imaging fractures. They are also used in airport security scanners and industrial quality control to check welds and structures without cutting them open.

Gamma rays are used in radiotherapy to kill cancerous tumour cells, to sterilise medical instruments, and as tracers in gamma cameras (a radioactive substance is injected into the patient and a detector maps where it concentrates in the body).

Which types of electromagnetic radiation are hazardous?

What do you think determines how dangerous a type of radiation is — its wavelength, its frequency, or its energy? The answer is all three (they are related), but the key concept is energy per photon.

At the low-frequency end — radio waves, microwaves, infrared, and visible light — radiation is generally safe at everyday intensities. Very intense infrared can burn skin, and extremely bright visible light can damage the retina, but these hazards require unusual exposure levels.

Ultraviolet is more concerning. UV-B and UV-C carry enough energy per photon to break chemical bonds in biological molecules. Prolonged skin exposure causes sunburn, premature skin ageing, and — by damaging the DNA in skin cells — can cause skin cancer.

X-rays and gamma rays are ionising radiation: each photon carries enough energy to remove electrons from atoms entirely, breaking chemical bonds and disrupting DNA. This can cause mutations leading to cancer. Medical X-rays are kept to the minimum dose needed for diagnosis. Workers in nuclear and medical settings wear film-badge dosimeters and are subject to strict dose limits.

The rule is straightforward: higher frequency → more energy per photon → greater potential hazard.

What is the difference between ionising and non-ionising radiation?

Non-ionising radiation covers radio waves, microwaves, infrared, and visible light. These photons do not carry enough energy to remove electrons from atoms; they can cause heating at high intensities but cannot directly damage DNA.

Ionising radiation covers ultraviolet (particularly UV-B and UV-C), X-rays, and gamma rays. These photons carry enough energy to ionise atoms — that is, to strip away electrons — which breaks chemical bonds and can damage DNA strands in living cells.

Because ionising radiation can cause mutations, it is the most dangerous category biologically. The precautions scale with the hazard: sunscreen absorbs UV before it reaches skin; lead aprons and concrete walls block X-rays and gamma rays in hospitals and nuclear plants; protective eyewear shields against high-powered lasers.

Understanding the ionising/non-ionising boundary is essential for the UK KS3 programme of study and forms the foundation for GCSE work on radiation risk.

How does the electromagnetic spectrum relate to the KS3 curriculum?

The DfE KS3 science programme of study requires pupils to know the properties shared by all electromagnetic waves, the uses and hazards of each region of the spectrum, and to understand that ionising radiation can damage living tissue. This knowledge underpins GCSE Combined Science and GCSE Physics, where questions on electromagnetic spectrum appear in every specification, including AQA and OCR.

The key exam skills are: ordering the seven regions by wavelength and by frequency; explaining why high-frequency radiation is more dangerous; and matching uses to the correct region. A common error is placing microwaves between infrared and ultraviolet — keep the mnemonic R-M-I-V-U-X-G (Radio, Micro, Infrared, Visible, UV, X-ray, Gamma) firmly in mind.

Frequently asked questions

Why does the electromagnetic spectrum include visible light?

Visible light is just one narrow band of the electromagnetic spectrum — the part human eyes are sensitive to, spanning roughly 400 nm (violet) to 700 nm (red). All other regions have the same wave nature as visible light; our eyes simply cannot detect them. The reason humans evolved to see this specific band is because it is the peak output of the Sun, which penetrated Earth's atmosphere during our evolution.

What is the difference between radio waves and gamma rays?

Both are electromagnetic waves travelling at the speed of light, but they sit at opposite ends of the spectrum. Radio waves have the longest wavelengths (>0.1 m) and lowest frequencies, so each wave carries very little energy — they pass harmlessly through the body. Gamma rays have the shortest wavelengths (<0.01 nm) and highest frequencies, so each ray carries enormous energy, enough to ionise atoms and damage DNA. Frequency is what determines the danger.

Can electromagnetic waves travel through space?

Yes — unlike sound, which is a mechanical wave that needs a medium (particles) to travel through, all electromagnetic waves are self-propagating oscillations of electric and magnetic fields. They can travel through the near-perfect vacuum of space, which is why we receive light, radio signals, X-rays, and gamma rays from distant stars and galaxies. This is also why you can see the Sun and stars but cannot hear them.

Why is ultraviolet light dangerous if visible light is safe?

Ultraviolet light sits just beyond the violet end of visible light and has a higher frequency (shorter wavelength), which means each UV photon carries more energy. That extra energy is just enough to ionise biological molecules, particularly the DNA in skin cells. This can cause mutations that lead to skin cancer. Visible light photons do not carry enough energy to ionise DNA, which is why a bright lamp is safe but prolonged UV exposure is not. Sunscreens work by absorbing or reflecting UV before it reaches skin cells.

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