Science
What is the electromagnetic spectrum, in order?
Quick answer
The electromagnetic spectrum is the whole family of electromagnetic waves: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. All travel at the speed of light.
In one sentence
The electromagnetic spectrum is the full range of electromagnetic waves, from long, low-frequency radio waves to short, high-frequency gamma rays, with visible light as a small slice in the middle.
Explained
Light is a wave, but it is only one member of a much bigger family. All the members are electromagnetic waves: vibrations of electric and magnetic fields that carry energy from one place to another. They are transverse waves, they need no material to travel through, and in a vacuum they all travel at the same speed, the speed of light, about 3.0 × 10⁸ m/s (300,000 km every second). That is how light and heat from the Sun cross empty space to reach us.
sun
phone
rainbow
The waves differ in wavelength (the distance from one peak to the next) and frequency (how many waves pass a point each second, in hertz, Hz). Going along the spectrum, wavelength gets shorter, frequency gets higher, and each little packet of energy (a photon) carries more energy. In order of increasing frequency:
radio waves → microwaves → infrared → visible light → ultraviolet → X-rays → gamma rays
A popular way to remember it: "Raging Martians Invaded Venus Using X-ray Guns". Inside the visible part, red has the longest wavelength and violet the shortest (ROYGBIV, red to violet).
Speed, frequency and wavelength are linked by the wave equation:
wave speed = frequency × wavelength, v = f × λ
Because the speed is fixed, a higher frequency always means a shorter wavelength. These numbers are huge or tiny, so we write them in standard form and use prefixes: kHz = 10³ Hz, MHz = 10⁶ Hz, GHz = 10⁹ Hz, and nm = 10⁻⁹ m.
Is it dangerous? It depends on the frequency. Radio waves, microwaves, infrared and visible light are non-ionising: they cannot knock electrons out of atoms. The main known effect of strong microwaves or infrared is heating, and phones and wifi use low power. Ultraviolet can damage skin cells and eyes, which is why we use sunscreen; its highest-frequency part is weakly ionising. X-rays and gamma rays are ionising: they can damage DNA, so doses are kept as low as possible and staff use shielding.
The spectrum was pieced together over about a century. William Herschel found infrared in 1800 with a thermometer placed beyond the red end of a spectrum; Johann Ritter found ultraviolet in 1801; James Clerk Maxwell predicted electromagnetic waves in the 1860s; Heinrich Hertz made radio waves in 1887; Wilhelm Röntgen discovered X-rays in 1895; and Paul Villard identified gamma rays, from radioactivity, in 1900.
Examples
| Wave | Typical wavelength | Common uses | Notes |
|---|---|---|---|
| Radio waves | about 1 m to many km | radio and TV broadcasts | longest wavelength |
| Microwaves | about 1 mm to 1 m | phones, wifi, satellites, microwave ovens | heat water in food |
| Infrared | about 700 nm to 1 mm | remote controls, thermal cameras, grills, optical fibres | felt as heat |
| Visible light | about 400 nm (violet) to 700 nm (red) | seeing, photography | the only part our eyes detect |
| Ultraviolet | about 10 nm to 400 nm | security marks, sterilising water, fluorescent lamps | causes sunburn |
| X-rays | about 0.01 nm to 10 nm | images of bones, airport security | ionising |
| Gamma rays | less than about 0.01 nm | radiotherapy, sterilising medical equipment | ionising, from atomic nuclei |
Worked example: an FM radio station broadcasts at 100 MHz. λ = v ÷ f = (3.0 × 10⁸ m/s) ÷ (1.0 × 10⁸ Hz) = 3.0 m.
Common mistakes
- Forgetting to convert prefixes. 100 MHz is 1.0 × 10⁸ Hz and 500 nm is 5.0 × 10⁻⁷ m. Convert to Hz and metres before using v = f × λ.
- "Higher frequency waves travel faster." In a vacuum they all travel at the same speed; higher frequency only means shorter wavelength.
- "Sound is part of the spectrum." Sound is a mechanical wave that needs a material. It travels at about 340 m/s in air and not at all in space.
- "All radiation is radioactive and harmful." Radio waves, microwaves and visible light are radiation too, but non-ionising. Only the high-frequency end is ionising.
Step by step
Step by step
- Write the equationV = f × λ. Rearrange if you need to: f = v ÷ λ or λ = v ÷ f.
- Convert the unitsFrequency in Hz, wavelength in m: MHz × 10⁶, GHz × 10⁹, nm × 10⁻⁹.
- Use the speed of lightFor electromagnetic waves in air or a vacuum, v = 3.0 × 10⁸ m/s.
- Calculate in standard formDivide the numbers, then subtract the powers of ten: 10⁸ ÷ 10⁹ = 10⁻¹.
- Check it makes senseRadio wavelengths come out in metres, visible light in hundreds of nanometres, X-rays far smaller.
Practice questions
The electromagnetic spectrum and v = f × λ
Try each one, then check your answer.
- Put these in order of increasing frequency: X-rays, radio waves, visible light, microwaves.
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Radio waves, microwaves, visible light, X-rays - Which has the longer wavelength, infrared or ultraviolet?
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Infrared - What is the speed of all electromagnetic waves in a vacuum?
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About 3.0 × 10⁸ m/s - Write 2.4 GHz in hertz, in standard form.
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2.4 × 10⁹ Hz - Find the wavelength of a 2.4 GHz wifi signal.
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λ = (3.0 × 10⁸) ÷ (2.4 × 10⁹) = 0.125 m (12.5 cm) - A microwave oven uses 2.45 GHz. Find the wavelength.
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λ = (3.0 × 10⁸) ÷ (2.45 × 10⁹) ≈ 0.12 m (about 12 cm) - Red light has a wavelength of 700 nm. Find its frequency.
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700 nm = 7.0 × 10⁻⁷ m; f = (3.0 × 10⁸) ÷ (7.0 × 10⁻⁷) ≈ 4.3 × 10¹⁴ Hz - Violet light has a wavelength of 400 nm. Find its frequency.
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f = (3.0 × 10⁸) ÷ (4.0 × 10⁻⁷) = 7.5 × 10¹⁴ Hz - A wave has f = 5.0 × 10¹⁴ Hz and λ = 6.0 × 10⁻⁷ m. Find its speed.
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v = 5.0 × 10¹⁴ × 6.0 × 10⁻⁷ = 30 × 10⁷ = 3.0 × 10⁸ m/s - An X-ray has a wavelength of 1.0 × 10⁻¹⁰ m. Find its frequency.
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f = (3.0 × 10⁸) ÷ (1.0 × 10⁻¹⁰) = 3.0 × 10¹⁸ Hz - A gamma ray has a frequency of 3.0 × 10²⁰ Hz. Find its wavelength.
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λ = (3.0 × 10⁸) ÷ (3.0 × 10²⁰) = 1.0 × 10⁻¹² m - A long-wave radio signal has a wavelength of 1,500 m. Find its frequency in kHz.
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f = (3.0 × 10⁸) ÷ 1,500 = 2.0 × 10⁵ Hz = 200 kHz - Light from the Sun travels 1.5 × 10¹¹ m to Earth. How long does it take?
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t = (1.5 × 10¹¹) ÷ (3.0 × 10⁸) = 500 s (about 8 minutes 20 seconds) - Name the three ionising parts of the spectrum.
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Ultraviolet (the high-frequency part), X-rays and gamma rays
Word problems
Electromagnetic waves in real life
Try each one, then check your answer.
- A mobile phone signal has a frequency of 900 MHz. What is its wavelength?
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λ = (3.0 × 10⁸) ÷ (9.0 × 10⁸) ≈ 0.33 m (about 33 cm) - A satellite is 36,000 km above the ground. How long does a signal take to reach it?
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36,000 km = 3.6 × 10⁷ m; t = (3.6 × 10⁷) ÷ (3.0 × 10⁸) = 0.12 s - Lightning strikes 3 km away. Why do you see it about 9 seconds before you hear it?
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Light takes 3,000 ÷ (3.0 × 10⁸) = 0.00001 s, but sound at about 340 m/s takes 3,000 ÷ 340 ≈ 8.8 s. - A rescue team finds people in the dark with a special camera. Which waves does it detect, and why does it work?
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Infrared: warm bodies give out more infrared than cooler surroundings. - Why does sunscreen protect your skin, and which waves does it block?
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It absorbs or reflects ultraviolet, which can damage skin cells and cause sunburn.
Questions people ask
What is the electromagnetic spectrum in order?
From lowest to highest frequency (longest to shortest wavelength): radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
How do you remember the electromagnetic spectrum?
Use a mnemonic for the first letters R, M, I, V, U, X, G, such as "Raging Martians Invaded Venus Using X-ray Guns". Then remember that frequency and energy go up along the list while wavelength goes down.
What is the electromagnetic spectrum made of?
Electromagnetic waves: linked vibrations of electric and magnetic fields. In physics they can also be described as streams of tiny energy packets called photons.
How is the electromagnetic spectrum organised?
By wavelength or frequency, which is the same thing in reverse because v = f × λ and the speed is fixed. Energy per photon rises with frequency.
Why is the electromagnetic spectrum important?
Almost all our communication (radio, TV, phones, wifi) and much of medicine and astronomy rely on it. Telescopes for radio, infrared and X-rays show parts of the universe our eyes cannot see.
Is visible light part of the electromagnetic spectrum?
Yes. It is the narrow band from about 400 nm (violet) to 700 nm (red) that our eyes can detect.
Quick quiz
The electromagnetic spectrum
Check what you know with 5 quick questions.
Question 1 of 5
Which list is in order of increasing frequency?
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Related topics
Light
Light is a form of energy that travels in straight lines, very fast. We see things when light from them enters our eyes, either because they make light or because they reflect it.
Radioactivity
Radioactivity is when unstable atomic nuclei break down and give out radiation: alpha, beta or gamma. Half-life is the time it takes for half of the radioactive nuclei in a sample to decay.
The Sun
The Sun is a star at the centre of our solar system. It is a huge ball of hot gas that gives us light and heat, and it is about 150 million kilometres from Earth.
Scientific notation
Scientific notation writes very large or very small numbers as a number from 1 to 10 times a power of 10. For example, 3,500,000 is 3.5 × 10⁶. In the UK it is called standard form.
Waves
A wave carries energy from place to place without carrying matter. Waves can be transverse or longitudinal, and wave speed = frequency × wavelength (v = fλ).
Acceleration
Acceleration is how quickly velocity changes. a = (v − u) ÷ t, the change in velocity divided by the time taken, measured in metres per second squared (m/s²).