Edexcel IGCSE Physics · Spec 3.10-3.14
The Electromagnetic Spectrum
The electromagnetic spectrum, its order, and the uses and dangers of each type.
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The Electromagnetic Spectrum
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Explained
Seven kinds of wave, one family
The electromagnetic spectrum is a continuous range of waves that share three properties. They are all transverse, they all transfer energy, and in a vacuum they all travel at the same speed, three times ten to the eight metres per second.
What differs is the wavelength and the frequency, and those two always move in opposite directions: as wavelength decreases across the spectrum, frequency increases.
The order
From longest wavelength to shortest: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.
Frequency and energy run the other way, so gamma has the highest frequency and carries the most energy per wave, and radio the least.
Visible light is a narrow band in the middle, and it is the only part we can see. Its own order, from longest wavelength to shortest, is red, orange, yellow, green, blue, indigo, violet, which sits neatly between infrared on one side and ultraviolet on the other.
A use for each
- Radio waves: broadcasting radio and television.
- Microwaves: cooking, mobile phones and satellite communication.
- Infrared: heating, remote controls, thermal imaging and optical fibres.
- Visible light: seeing, photography and optical fibres.
- Ultraviolet: fluorescent lamps, security marking and sterilising water.
- X-rays: medical imaging and security scanners.
- Gamma rays: sterilising equipment, detecting and treating cancer.
The dangers
Ultraviolet, X-rays and gamma rays are ionising, meaning they carry enough energy to remove electrons from atoms. Inside a cell that can damage or mutate DNA, which can cause cancer.
Microwaves and infrared are not ionising, so their hazard is heating: internal heating of tissue and skin burns. Radio waves are the least harmful of all.
The pattern is worth stating rather than memorising. Danger increases with frequency, because frequency is what determines the energy each wave carries.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme includes a tick box question listing four statements about sound waves, with two correct: that sound waves are longitudinal, and that they can be reflected and refracted. The two that are wrong are that sound can travel through a vacuum and that sound is part of the electromagnetic spectrum.
Both wrong statements are about the boundary of this topic. Electromagnetic waves are transverse and need no medium; sound is longitudinal and cannot cross a vacuum. Sound is not part of this family at all, however often the two are taught together.
The marking instruction is worth noting too. The mark scheme says that if more than two boxes are ticked, one mark is deducted for each incorrect tick.
So ticking everything to be safe is worse than ticking nothing. Where a question tells you how many answers it wants, giving more than that is actively penalised, and this is one of the few places in the paper where a wrong answer costs you rather than simply earning nothing.
Why they all travel at the same speed
All electromagnetic waves are oscillating electric and magnetic fields, so in a vacuum they all propagate at the same speed regardless of wavelength.
That constancy is what makes the wave equation so useful here. Since speed equals frequency times wavelength, and the speed is fixed, knowing either one gives you the other, and a longer wavelength must mean a lower frequency.
In a material they do slow down, and by different amounts, which is why a prism separates white light into a spectrum.
Spec 3.10-3.14
What you need to know
- List the seven parts of the EM spectrum in order
- Give a use for each type of wave
- Describe the dangers of some EM waves
Active recall
Quick check
Answer each question before opening the answer.
List the EM spectrum from longest to shortest wavelength.
Radio, microwave, infrared, visible light, ultraviolet, X-rays, gamma rays.
What do all electromagnetic waves have in common?
They are transverse and travel at the same speed (the speed of light) in a vacuum.
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