The Electromagnetic Spectrum

Seven regions of the same family of waves, differing only in wavelength and frequency. Most questions test whether you know the order, the trends, and one use and danger for each region.

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1. What they all share

All electromagnetic waves:

• are transverse waves
• transfer energy
• can travel through a vacuum
• travel at the speed of light in a vacuum: 3 × 10⁸ m/s
• obey v = f λ

EM waves are TRANSVERSE, not longitudinal. Confusing them with longitudinal waves (like sound) was a recorded error. Sound is not an EM wave at all — it needs a medium, and EM waves do not.

All EM waves travel at the same speed in a vacuum, regardless of region. Radio waves and gamma rays travel equally fast — what differs is frequency and wavelength.

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2. The order

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

Region	Wavelength	Frequency	Energy
Radio	longest	lowest	lowest
Microwave	↓	↑	↑
Infrared	↓	↑	↑
Visible	↓	↑	↑
Ultraviolet	↓	↑	↑
X-ray	↓	↑	↑
Gamma	shortest	highest	highest

A mnemonic that works: Rock Music Is Very Useful for eXtremely Great parties.

Going left to right: wavelength DECREASES, frequency INCREASES, energy INCREASES. Frequency and energy move together; wavelength moves opposite to both.

Learn the order in one direction and stick to it. Not knowing the order was a recorded error, and every “which has the highest frequency?” question depends on it.

Radio waves have the LOWEST frequency and the LONGEST wavelength. Gamma is the opposite extreme.

Visible light is a tiny band in the middle. In order of increasing frequency: red, orange, yellow, green, blue, indigo, violet (ROYGBIV, or VIBGYOR read backwards).

Red has the longest wavelength and lowest frequency; violet the shortest and highest.

VIBGYOR is the visible part only — it is not the whole electromagnetic spectrum. Confusing the two was a recorded error. Visible light is one of seven regions.

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3. Uses and dangers

Region	Uses	Dangers
Radio	broadcasting, TV, radio astronomy	none significant
Microwave	cooking, mobile phones, satellite communication	internal heating of body tissue
Infrared	remote controls, thermal imaging, grills, optical fibres, intruder alarms	skin burns
Visible	seeing, photography, illumination, optical fibres	—
Ultraviolet	sterilising water, security marking on banknotes, sun beds, detecting forgeries, fluorescent lamps	skin cancer, eye damage
X-ray	medical imaging of bones, airport security scanning	cell mutation, cancer
Gamma	sterilising medical equipment and food, cancer treatment (radiotherapy), medical tracers	cell mutation, cancer

Points that are examined repeatedly:

Remote controls use INFRARED, not radio waves — a recorded error.

Banknote security marking uses ULTRAVIOLET, not radio waves — also recorded. UV makes special inks fluoresce.

Microwaves cook food by heating water molecules inside it; infrared heats the surface (as in a grill or toaster).

Both gamma rays and UV are used for sterilisation, but for different things: UV sterilises water and surfaces, while gamma sterilises medical equipment and food because it penetrates sealed packaging. Both answers are correct in their own context — check what the question is sterilising.

The dangers increase with frequency. The high-frequency end (UV, X-ray, gamma) is ionising — it can damage cells and cause mutations and cancer. The low-frequency end is far less harmful.

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4. Calculations

Use v = f λ with v = 3 × 10⁸ m/s.

Example: a radio wave has a wavelength of 300 m. Find its frequency.

• f = v/λ = (3 × 10⁸) ÷ 300 = 1 × 10⁶ Hz (1 MHz)

Example: an X-ray has frequency 3 × 10¹⁸ Hz. Find its wavelength.

• λ = v/f = (3 × 10⁸) ÷ (3 × 10¹⁸) = 1 × 10⁻¹⁰ m

Know the speed of light: 3 × 10⁸ m/s. Forgetting it was a recorded error and makes every calculation impossible.

Work in standard form and take care with the powers of ten. Convert kHz and MHz to Hz first.

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5. Communication

• Radio waves — long-range broadcasting; long wavelengths diffract around hills and buildings
• Microwaves — satellite communication and mobile phones; they pass through the atmosphere
• Infrared and visible light — optical fibres, using total internal reflection

Longer wavelengths diffract more, which is why radio reaches into valleys while microwaves need line-of-sight or a satellite.

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6. Mistakes that cost marks

Not knowing the order of the spectrum.

Saying EM waves are longitudinal.

Thinking different regions travel at different speeds in a vacuum.

Confusing VIBGYOR with the whole spectrum.

Getting the wavelength/frequency trend backwards.

Saying remote controls use radio waves — they use infrared.

Saying banknote security uses radio waves — it uses UV.

Forgetting the speed of light.

Mishandling standard form in calculations.

Giving a use without the matching region, or a danger for the wrong end of the spectrum.

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Frequently asked questions

What is the order of the electromagnetic spectrum? Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.

Which has the longest wavelength? Radio waves — they also have the lowest frequency and energy.

Which has the highest frequency? Gamma rays.

Do all EM waves travel at the same speed? Yes — 3 × 10⁸ m/s in a vacuum.

Are EM waves transverse or longitudinal? Transverse.

What is used in a TV remote control? Infrared.

What is used for security marks on banknotes? Ultraviolet.

What are the dangers of ultraviolet? Skin cancer and eye damage.

Why are gamma rays dangerous? They are ionising and can cause cell mutation and cancer.

What is the visible spectrum order? Red, orange, yellow, green, blue, indigo, violet — increasing in frequency.

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Quick revision checklist

• I know all seven regions in order
• I have a mnemonic I can write down under pressure
• I know wavelength decreases and frequency and energy increase across the spectrum
• I know all EM waves are transverse and cross a vacuum
• I know the speed of light is 3 × 10⁸ m/s
• I can give a use for every region
• I can give a danger for every region
• I know remote controls use infrared and banknotes use UV
• I know the visible spectrum order
• I can use v = fλ in standard form
• I know why longer wavelengths diffract more

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These notes cover the electromagnetic spectrum in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.