3.12-3.13 - Uses and risks of electromagnetic radiation
Electromagnetic radiation is useful because different parts of the spectrum interact with matter in different ways. Some waves are good at carrying information, some are absorbed and cause heating, and some can pass through materials or kill microorganisms. The same properties that make a radiation useful can also make excessive exposure dangerous, so each use needs a sensible control of risk.
Choosing radiation for a job
The electromagnetic spectrum is not just a list to memorise. Each region is chosen for a job because of how it is transmitted, absorbed, reflected, or detected.
For this lesson, use this simple pattern:
| Property of the radiation | Why it can be useful |
|---|---|
| It travels through air or space and can be received by an aerial or dish. | It can carry information for broadcasting and communications. |
| It is absorbed by water-rich or surface tissue. | It can heat materials, but can also heat body tissue. |
| It can be detected by a camera, sensor, or photographic detector. | It can form images or reveal things the eye cannot see directly. |
| It has high energy and penetrates some materials. | It can image internal structures or kill microorganisms, but may damage cells. |
When an exam question asks you to explain a use, do not only name the radiation. Link the radiation to the useful property and then to the job it does.
For example, microwaves are used for satellite transmissions because they can travel through the atmosphere to and from a satellite and can be used to carry an information signal. Gamma rays are used for sterilising medical equipment because they can penetrate packaging and kill microorganisms.
A strong explanation links radiation type, useful property, and application.
Communications and images
Radio waves are used for broadcasting and communications. A transmitter makes a radio-wave signal that carries information, such as sound, pictures, or digital data. A receiver aerial picks up the signal and equipment converts it back into useful information.
This is why radio waves are used for radio and television broadcasting, two-way radio links, and other communication systems. The exact system may be different, but the basic idea is the same: information is carried by an electromagnetic wave from transmitter to receiver.
Microwaves are also used for communications, especially satellite transmissions. A dish can send a narrow microwave beam to a satellite, and another dish can receive the signal. Microwaves are suitable because they can pass through the atmosphere well enough for communication with satellites.
Visible light is used for photography because cameras use visible light reflected or emitted by objects to form an image on a detector. Visible light is also used in optical fibres. In an optical fibre, pulses of light travel along a thin transparent fibre, so information can be sent as a sequence of light pulses.
Heating and fluorescence
Microwaves are used for cooking because they are absorbed by water molecules in food. The oscillating electric field turns polar water molecules; interactions with neighbouring molecules transfer energy to the thermal store of the food, so it heats up. At this level, absorption and heating are the required link.
Infrared radiation is used in heaters because infrared is absorbed by surfaces and transfers energy by heating. A heater does not need to warm all the air first; infrared can be absorbed by your skin or by the surface of an object, making it feel warm.
Infrared is also used in night vision equipment. Warm objects, including people and animals, emit infrared radiation. A night vision sensor or thermal camera detects that infrared radiation and produces an image, even when there is too little visible light for normal sight.
Ultraviolet radiation is used in fluorescent lamps. Inside the lamp, ultraviolet radiation is produced and absorbed by a fluorescent coating. The coating then emits visible light. The useful output is visible light, but ultraviolet radiation is part of the process that makes the lamp work.
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