(c) The electromagnetic spectrum
- Syllabus
- 2024
- Topic
- —
- Level
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Learning objectives
3.10Electromagnetic spectrumKnow that light is part of a continuous electromagnetic spectrum that includes radio, microwave, infrared, visible, ultraviolet, x-ray and gamma ray radiations, and that all these waves travel at the same speed in free space3.11EM spectrum orderKnow the order of the electromagnetic spectrum in terms of decreasing wavelength and increasing frequency, including the colours of the visible spectrum3.12Uses of EM radiationExplain some of the uses of electromagnetic radiations, including:• radio waves: broadcasting and communications• microwaves: cooking and satellite transmissions• infrared: heaters and night vision equipment• visible light: optical fibres and photography• ultraviolet: fluorescent lamps• x-rays: observing the internal structure of objects and materials, including for medical applications• gamma rays: sterilising food and medical equipment3.13Risks of EM radiationExplain the harmful effects of excessive electromagnetic-wave exposure: microwave internal heating, infrared skin burns, ultraviolet surface-cell damage and blindness, and gamma-ray cancer and mutation; describe simple protective measures.
Meet the electromagnetic spectrum
Visible light is one small region of a continuous electromagnetic spectrum. The seven named regions are radio waves, microwaves, infrared, visible light, ultraviolet, x-rays and gamma rays.
Continuous means that electromagnetic wavelengths form an unbroken range. The named regions are useful bands within that range, not separate kinds of disturbance with gaps between them.
Every electromagnetic wave travels at the same speed in free space: 3.0×108 m/s. A radio wave and a gamma ray can have very different frequencies and wavelengths while sharing this free-space speed.
The common speed applies in free space. Electromagnetic waves can travel at different, lower speeds in materials, so do not apply the free-space statement to every medium.
Order the electromagnetic spectrum
Across the electromagnetic spectrum, decreasing wavelength means increasing frequency because all regions share the same free-space speed and v=fλ.
| Direction | Ordered spectrum |
|---|---|
| wavelength decreases; frequency increases | radio → microwave → infrared → visible → ultraviolet → x-ray → gamma |
| wavelength increases; frequency decreases | gamma → x-ray → ultraviolet → visible → infrared → microwave → radio |
Within visible light, the order from longer wavelength and lower frequency to shorter wavelength and higher frequency is red → orange → yellow → green → blue → indigo → violet.
Ultraviolet is beyond violet but is not a visible colour; infrared is beyond red but is not visible. ‘Higher in the spectrum’ is ambiguous, so always state whether frequency or wavelength is increasing.
Connect electromagnetic waves to their uses
An electromagnetic wave is chosen for a use because its interaction with matter, transmission or detection makes that job possible.
| Region | Required uses | Why it is useful |
|---|---|---|
| radio waves | broadcasting and communications | signals can carry information over large distances |
| microwaves | cooking; satellite transmissions | absorbed energy heats food; directed signals can pass through the atmosphere to and from satellites |
| infrared | heaters; night vision | readily transfers thermal energy; warm objects emit infrared that detectors can sense |
| visible light | optical fibres; photography | can be guided along transparent fibres; cameras detect it to form photographs |
| ultraviolet | fluorescent lamps | fluorescent materials absorb ultraviolet and emit visible light |
| x-rays | seeing internal structures, including medical imaging | pass through some materials and soft tissue more readily than denser material such as bone |
| gamma rays | sterilising food and medical equipment | penetrating radiation kills microorganisms |
A region may have other valid uses, but these are the pairings required here. The use is not explained by naming the wave alone: connect the wave's behaviour to what the device or process needs.
Match electromagnetic hazards to protection
Excessive exposure to electromagnetic radiation can harm the body. Risk is reduced by limiting exposure time, increasing distance where practical, and placing suitable shielding between the source and people.
| Radiation | Required harmful effect | Simple protective measures |
|---|---|---|
| microwaves | internal heating of body tissue | metal shielding and safety interlocks; keep away from a leaking source |
| infrared | skin burns | heat-resistant shielding or clothing; increase distance and limit exposure time |
| ultraviolet | damage to surface cells and blindness | cover skin, use sunscreen and UV-blocking eye protection; limit time in strong sunlight |
| gamma rays | cell mutation and cancer | dense shielding, remote handling, greater distance and the shortest practical exposure time |
Protection must interrupt the exposure route: shielding absorbs or blocks radiation, distance reduces the radiation reaching the body, and shorter exposure reduces the total received.
Do not swap the named hazards: microwaves cause internal heating, infrared causes surface burns, ultraviolet damages surface cells and eyes, and gamma radiation can cause mutation and cancer.