Uses across the spectrum

Radio waves

• Radio and television communication: A transmitter sends an electromagnetic signal carrying information. A receiver detects it and reconstructs the programme. Producing sound at the receiver requires a loudspeaker; the travelling radio signal itself is not sound.
• Astronomy: Radio telescopes detect radio emissions arriving from astronomical objects. The detected signals provide information about those objects; the telescope does not have to illuminate every object with its own transmitted beam.
• RFID tags: Radio-frequency identification uses radio signals between a tag and a reader to exchange identification information, for example to identify a tagged item.

Microwaves

• Mobile phones: Mobile communication links can use microwaves to carry information between a phone and a base station. A receiver detects the signal rather than using it primarily to heat an object.
• Microwave ovens: Food absorbs microwave energy, increasing its internal energy and temperature. This is heating by absorbed radiation; it does not make the food radioactive or mean that all food heats uniformly from the centre outwards.
• Satellite television: Microwave signals carry information between ground equipment and a satellite, and a receiving dish collects the downlink. Suitable frequency bands are chosen for transmission through the atmosphere; not every microwave frequency passes through equally well.

Infrared

• Infrared remote controllers: A remote sends coded infrared pulses to a detector in the appliance. Different pulse patterns can represent different commands.
• Intruder alarms: A passive infrared sensor detects changes in incoming infrared from its surroundings, such as when a warm body moves across its view. In this type of alarm, the sensor receives the detecting radiation rather than emitting a beam.
• Thermal imaging: A camera detects infrared emission from a scene and builds a displayed image. Its colours are assigned to detector information; they are not the visible colour of infrared radiation itself.

The same region of the spectrum can therefore serve different purposes. For a remote, the detector reads a deliberate code. For a passive intruder alarm, the important observation is a change in incoming infrared as the scene changes.

Visible light

• Photography: A camera records visible light from a scene on film or a sensor. The detected pattern forms the photograph.
• Optical fibres in medicine: A fibre-optic endoscope can carry visible illumination to an internal surface and return an image through a bundle of fibres, allowing that surface to be viewed.
• Optical fibres in telecommunications: Light signals carry information along a fibre. A short link can use a visible red source. Many telecommunications fibres instead use infrared wavelengths, so fibre communication is not restricted to visible light.

Ultraviolet

• Sunbeds: Sunbeds use ultraviolet radiation to tan skin. This exposure can damage cells and increases skin-cancer risk; being an application does not make the exposure harmless.
• Bank-note authentication: Fluorescent security features emit visible light when illuminated with ultraviolet. Their observed pattern can be checked as part of authentication; the eye sees the emitted visible light, not the UV itself.
• Disinfecting water: Ultraviolet can damage the genetic material of microorganisms, preventing them from reproducing. Effective disinfection depends on enough radiation reaching the microorganisms under the chosen treatment conditions.

X-rays

• Medical radiology: Different tissues absorb or transmit different amounts of X-radiation. A detector records the transmitted pattern, revealing internal structure. The eye does not directly see the X-rays.
• Security screening: Differences in X-ray transmission help show objects inside baggage or packages without opening them.
• Industrial defect detection: A crack or void can change the material or thickness along the X-ray path. The altered transmitted pattern can reveal a defect inside a component.

Gamma rays

• Sterilising food: Gamma irradiation can destroy microorganisms. Treatment can be used for sterilisation or to reduce microbial contamination under chosen conditions; not every irradiated food is completely sterile.
• Detecting cancer: A suitable radioactive tracer emits gamma radiation that is detected outside the body. An image of its distribution can help locate abnormal tissue, including some cancers; it is not a universal test that identifies every cancer.
• Treating cancer: Gamma radiation can be directed at a tumour to damage cancer cells. Healthy tissue can also be affected, so the useful effect depends on controlling the exposure rather than making the radiation harmless.