The electromagnetic spectrum is the full range of electromagnetic waves, ordered by wavelength and frequency. From longest wavelength to shortest it runs: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. All of them are transverse waves, all of them can travel through a vacuum, and all of them travel at the same speed in a vacuum, about 3.0 × 10⁸ m/s. What changes along the spectrum is wavelength and frequency, and with them energy: as wavelength gets shorter, frequency and energy go up. Ultraviolet, X-rays and gamma rays carry enough energy to ionise atoms, which is why they are the dangerous end.

This guide covers what a GCSE physics course (AQA, Edexcel, OCR and equivalent) expects, in the order you would draw it.

One family, seven names

The first thing to draw is not seven separate things but one continuous wave that gets squashed from left to right. The names are labels people put on ranges of that one spectrum, mostly according to how the waves are made and detected. The boundaries are fuzzy: the shortest microwaves and the longest infrared overlap, and so do high energy X-rays and gamma rays.

What every electromagnetic wave has in common:

• It is a transverse wave: the oscillations (of electric and magnetic fields) are at right angles to the direction the wave travels.
• It transfers energy from a source to an absorber, not matter.
• It needs no medium. Light reaches us from the Sun through the vacuum of space; sound cannot.
• It travels at the same speed in a vacuum: 3.0 × 10⁸ m/s, usually written c. In air it is almost exactly the same. In glass or water it slows down, which causes refraction.

The spectrum in one table

Wavelengths below are typical orders of magnitude, which is what GCSE exams ask for.

Two patterns in this table are worth drawing as arrows above and below the whole spectrum. Left to right, wavelength decreases. Left to right, frequency and energy increase. Every "explain why" question about danger comes back to the second arrow.

Where each kind of wave comes from

Higher courses ask this, and it also makes the video easier to follow.

• Radio waves are produced by oscillating electrons in an electrical circuit, such as an aerial. When radio waves are absorbed by another aerial they make the electrons there oscillate, creating an alternating current at the same frequency. That is how a radio receives a station.
• Infrared is given off by everything that is warm. Hotter objects emit more of it.
• Visible light and ultraviolet come from changes in the energy levels of electrons in atoms, which is why very hot objects (and the Sun) give off both.
• X-rays are produced when fast electrons are stopped suddenly, as in an X-ray tube. They come from outside the nucleus.
• Gamma rays come from changes inside the nucleus of an atom, during radioactive decay.

The X-ray and gamma ray distinction is a common trap. They overlap in wavelength. They are named by where they come from, not only by how short their wavelength is.

The wave equation, with worked examples

All waves obey the same equation:

wave speed = frequency × wavelength, or v = f λ

For electromagnetic waves in a vacuum the speed is c, so c = f λ, with c = 3.0 × 10⁸ m/s, frequency in hertz (Hz) and wavelength in metres (m). Because c is fixed, frequency and wavelength are inversely proportional: double one and the other halves.

Worked example 1: a longwave radio station. For decades BBC Radio 4 was broadcast on longwave at 198 kHz. What is the wavelength of that signal?

1. Convert to hertz: 198 kHz = 198 000 Hz = 1.98 × 10⁵ Hz.
2. Rearrange: λ = c ÷ f.
3. Substitute: λ = 3.0 × 10⁸ ÷ 1.98 × 10⁵.
4. Answer: λ ≈ 1500 m, about one and a half kilometres.

Worked example 2: a microwave oven. Microwave ovens usually work at 2.45 GHz.

1. 2.45 GHz = 2.45 × 10⁹ Hz.
2. λ = 3.0 × 10⁸ ÷ 2.45 × 10⁹ ≈ 0.12 m, about 12 cm.

Worked example 3: green light. Green light has a wavelength of about 550 nm. What is its frequency?

1. 550 nm = 550 × 10⁻⁹ m = 5.5 × 10⁻⁷ m.
2. f = c ÷ λ = 3.0 × 10⁸ ÷ 5.5 × 10⁻⁷ ≈ 5.5 × 10¹⁴ Hz.

Most lost marks in these questions come from units, not physics. Write the prefix table next to every calculation: kilo (k) = 10³, mega (M) = 10⁶, giga (G) = 10⁹, milli (m) = 10⁻³, micro (μ) = 10⁻⁶, nano (n) = 10⁻⁹.

Ionising and non-ionising radiation

Ionising radiation has enough energy per photon to knock electrons out of atoms, turning them into ions. In living cells that can break chemical bonds in DNA, which can kill cells or cause mutations that lead to cancer.

• Non-ionising: radio waves, microwaves, infrared, visible light, and lower energy ultraviolet. They can still harm you, mainly by heating (a burn, or a microwave oven cooking tissue), but they do not ionise.
• Ionising: higher energy ultraviolet, X-rays and gamma rays. GCSE courses usually group UV, X-rays and gamma rays together as the ionising, mutation-causing end.

The harm depends on the type of radiation and on the dose, which is measured in sieverts (Sv), or more usefully millisieverts (mSv). A single chest X-ray gives a small dose, which is why hospitals use them freely but still limit how many you have, and why the radiographer steps behind a screen.

Whether a wave is absorbed, transmitted or reflected depends on the material and the wavelength. Bone absorbs X-rays more than soft tissue does, which is what makes an X-ray image work: the bones show up white where the X-rays did not reach the detector.

How to remember the order

Pick a mnemonic for radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma, and write it at the top of every revision page until you stop needing it. Two common ones:

• Raging Martians Invaded Venus Using X-ray Guns
• Rich Men In Vegas Use eXpensive Gadgets

Then remember one anchor: you can stand in front of a radio transmitter all day, but not in front of a gamma source. Long, gentle waves on the left. Short, energetic waves on the right. Within visible light, the same rule holds: red has the longest wavelength and violet the shortest, which is why the next region past violet is called ultraviolet and the region below red is infrared.

Mistakes that lose easy marks

• Calling radio waves sound waves. A radio receives electromagnetic waves and converts them into sound. Sound is a longitudinal mechanical wave and cannot cross a vacuum.
• Saying higher frequency waves travel faster. Every electromagnetic wave travels at the same speed in a vacuum.
• Getting wavelength and frequency the same way round. As one goes up, the other goes down.
• Saying infrared is heat. Infrared is a wave that transfers energy. It heats whatever absorbs it.
• Saying microwaves are ionising, or that they make food radioactive. They are non-ionising. They heat food mainly because water molecules in the food absorb them.
• Forgetting units in c = f λ. kHz, MHz, GHz and nm must be converted before you substitute.
• Saying all ultraviolet is harmless because it is invisible, or that sunscreen blocks all of it. Sunscreen reduces exposure; it does not stop it entirely.

A scene plan for a four minute video

The spectrum is a comparison along a single line, so the video should keep that line on screen and move along it. About 600 words of narration:

1. A long horizontal wave drawn across the whole screen, stretched on the left and squashed on the right. Two arrows: wavelength decreases, frequency and energy increase.
2. Radio: an aerial, a house with a radio, the 198 kHz example worked on screen.
3. Microwaves: a satellite dish and a microwave oven, with 12 cm written beside the oven.
4. Infrared: a thermal camera image of a person and a TV remote.
5. Visible light: a prism splitting white light into colours, red to violet, with 700 nm and 400 nm labelled.
6. Ultraviolet: the Sun, a UV security pen and sunscreen.
7. X-rays: a hand X-ray with bones white, and a dashed line marking where ionising begins.
8. Gamma rays: a nucleus, a radiotherapy beam and a sterilised syringe.
9. The whole line again, with the mnemonic written under it, letter by letter.

Scene 7's dashed line does more work than any sentence. Students who remember where the line sits rarely get the danger questions wrong. For more on making a physics video that explains rather than recites, see how to make a physics explainer video that actually explains, and for the side by side layout used here, how to explain a comparison in a video.

Make your own version

1. 1

   Check your specification

   GCSE needs the order, typical wavelengths, uses, dangers and c = f x wavelength. Higher tier adds how radio waves are made and received. A level adds photons and E = hf. Write only what your course needs.

2. 2

   Choose your three worked examples

   One radio station from your own country, one microwave oven, one colour of visible light. Write each calculation out in full with the units converted.

3. 3

   Write one paragraph per scene

   Follow the scene plan above. About 600 words is four minutes at roughly 150 spoken words a minute.

4. 4

   Paste the script into openCanviz

   Choose Keep my wording so every number and unit is said exactly as written, set a target length, and pick the whiteboard style so the spectrum line builds left to right as you talk.

5. 5

   Check every number and label

   Pause on each scene. Drafted drawings can swap infrared and ultraviolet or put the wrong power of ten on a wavelength. Fix any wrong label in the editor.

Once it is made, use the video for revision the active way, as in how to turn class notes into a revision video: pause before each scene and say the next region, one use and one danger out loud. Watching the spectrum go past is not the same as being able to write it down.

Common questions

What are the seven types of electromagnetic waves in order? From longest wavelength and lowest frequency to shortest wavelength and highest frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.

Which electromagnetic waves are the most dangerous? Gamma rays and X-rays, followed by ultraviolet. They have the highest frequencies and energies, and they are ionising, so they can damage DNA in cells. Lower frequency waves can still cause harm through heating.

Do all electromagnetic waves travel at the same speed? Yes, in a vacuum all of them travel at about 3.0 × 10⁸ m/s. In materials such as glass or water they slow down, and the amount depends slightly on wavelength, which is why a prism separates colours.