Physics 5.20 - 5.22 - Uses and hazards of electromagnetic radiation

From waves to effects

Radio waves, microwaves, infrared, visible light, ultraviolet, x-rays and gamma rays are not seven unrelated substances. They are frequency ranges in one continuous electromagnetic spectrum, ordered from lowest to highest frequency:

radio → microwave → infrared → visible → ultraviolet → x-ray → gamma

Every electromagnetic wave transfers energy from a source. When it reaches matter, some of the radiation may be transmitted through the material, reflected away or absorbed. If radiation is absorbed, energy is transferred to the material. The consequence depends on the frequency, the amount absorbed and the material:

• absorbed microwave or infrared energy can increase the internal energy and temperature of matter;
• absorbed ultraviolet can cause chemical damage in surface cells and the eyes;
• x-rays and gamma rays can ionise atoms; the resulting molecular damage can cause mutations or other damage in cells.

This gives the required trend:

The word potential matters. A hazard is the ability to cause harm; risk is the chance and seriousness of harm in a particular exposure. Frequency affects the hazard, but actual risk also depends on the intensity or dose, exposure time, distance from the source, how much is absorbed and what shielding is present. A short, controlled medical x-ray can have a small justified risk, while excessive exposure to a powerful lower-frequency source can cause serious heating.

Do not use one rule for every part of the spectrum. Microwaves and infrared cause the named hazards mainly through heating; x-rays and gamma rays cause the named hazards through ionisation and cell damage. Ultraviolet sits between these regions and can damage surface cells and eyes even when no immediate heating is felt.

Radio, microwave and infrared uses

The lower-frequency regions are especially useful for carrying information and transferring thermal energy. A signal can carry information when a transmitter varies some feature of the wave and a receiver detects those variations. The wave transports energy and information; it does not carry a stream of air or sound from transmitter to receiver.

Microwave cooking and communication

In a microwave oven, some microwave energy penetrates into the food and is absorbed. This increases the internal energy of parts of the food, so their temperature rises; conduction then redistributes energy to other regions. It is inaccurate to say that every item cooks evenly "from the inside out" or that the whole food is penetrated equally.

Microwaves can also travel through the atmosphere and carry rapidly changing signals. A dish directs the signal towards a satellite, and another link sends information back to a receiver. Cooking and communication use the same radiation range for different outcomes: strong absorption is wanted in the food, while successful transmission is wanted along the communication path.

Infrared heating, sensing and signalling

Warm objects emit infrared radiation. A thermal imaging camera detects differences in the infrared reaching each part of its sensor and displays a temperature pattern. The camera does not need the object to glow visibly, so it can reveal warm bodies or hot components in darkness.

Infrared can also carry coded pulses. A television remote sends a short-range sequence to a receiver on the television. In an optical fibre, infrared pulses travel through glass and carry digital information over much longer distances; suitable infrared wavelengths are chosen because little energy is absorbed by the fibre. These are signal uses, not heating uses.

Worked selection: finding a person in darkness

A rescue team must locate a warm person in a dark room without illuminating it.

1. The required information is a map of temperature differences, not a visible photograph.
2. Warm bodies emit infrared radiation.
3. A thermal imaging camera detects the pattern of infrared reaching its sensor.
4. Therefore infrared is a suitable choice. A normal visible-light camera may fail because there is too little visible light to detect.

Visible and ultraviolet uses

Visible light is the range detected by human eyes. Its named uses all require a source, a pattern of light from an object or scene, and a detector.

• Vision: visible light from a source is reflected or emitted by objects and enters the eye, allowing the pattern to be detected.
• Photography: a camera lens directs visible light onto film or an electronic sensor, recording a pattern that forms an image.
• Illumination: a lamp produces visible light so objects can reflect enough light to be seen.

Ultraviolet is invisible to the human eye, but it can make some substances fluoresce. A fluorescent material absorbs ultraviolet and emits visible light. It is the emitted visible light that is seen, not the ultraviolet itself.

That one interaction explains several named uses:

• security marking: a mark that is hard to see in ordinary light glows visibly under a suitable ultraviolet lamp;
• fluorescent lamps: ultraviolet produced inside the lamp is absorbed by a fluorescent coating, which emits visible light;
• detecting forged bank notes: genuine notes contain designed fluorescent features that appear under ultraviolet while much of the background stays relatively dull.

Ultraviolet is also used for disinfecting water. A controlled UV source damages microorganisms or their genetic material so they cannot reproduce. The water must transmit enough ultraviolet for the required exposure to reach the microorganisms. The process neither heats the water to kill them nor makes the water radioactive.

Worked chain: checking a bank note

1. A suitable lamp sends ultraviolet towards the note.
2. A genuine fluorescent feature absorbs some ultraviolet.
3. The feature emits visible light in a designed pattern.
4. The checker sees that visible pattern and compares it with the expected security feature.