Uses of EM waves

The regions form a continuous spectrum rather than seven completely separate entities. In order from long wavelength and low frequency to short wavelength and high frequency, they are:

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

Radio waves: carrying information

Radio waves are used for radio and television broadcasting, telecommunications, Bluetooth and radar. A transmitter places information onto an electromagnetic carrier wave. A receiver detects the wave and converts the signal back into electrical information that can produce sound, pictures or data.

Radar sends out radio or microwave signals and detects their reflections from objects. This can be used to locate aircraft or ships.

Microwaves: communication and cooking

Microwaves are used in satellite communications and mobile-phone networks because suitable microwave frequencies can pass through the atmosphere and carry information. A satellite can receive a signal and relay it over a long distance.

In a microwave oven, water in food absorbs microwave energy. This transfers energy to the food and raises its temperature, so microwaves can be used for cooking. The wave is not itself “heat”; heating occurs when its energy is absorbed.

OCR suggests demonstrations in which microwaves light a bulb associated with a beaker of water, showing that the waves transfer energy, and in which a microwave emitter and absorber are used to investigate wave behaviour. The absorber receives energy from the emitter; placing or moving materials between them can show that different materials transmit or absorb microwaves differently.

Infrared: heating, imaging and control signals

Warm objects emit infrared radiation, and objects can absorb it and become warmer. Infrared is therefore used in electrical heaters and for cooking. Infrared cameras detect differences in the infrared radiation arriving from different parts of a scene, allowing warmer and cooler regions to be represented in an image.

Remote controls commonly send coded infrared pulses. Human eyes cannot see these pulses, but many phone-camera sensors can detect them. Pointing the remote at a phone camera while pressing a button can therefore make its infrared emitter appear to flash on the screen. The camera is detecting infrared and displaying a visible representation; the infrared has not become visible to the eye.

Visible light: seeing and transmitting data

Visible light enables vision and is used in lighting, photography and cameras. It is also used to carry information through optical fibres. Light is guided along a thin glass or plastic fibre, allowing data to follow a curved route rather than travelling only in a straight line through open air. This makes optical fibres useful for high-speed communications.

Ultraviolet: lamps, security marking and tanning

Ultraviolet radiation is used in some energy-efficient lamps. The ultraviolet is absorbed by a fluorescent coating, which then emits visible light.

Security markings on items such as banknotes may also fluoresce under ultraviolet radiation, helping genuine notes to be identified. Ultraviolet is used in sun tanning because it interacts with skin, although excessive exposure is hazardous; the hazards are considered separately.

X-rays: looking through the body

X-rays are used in medical imaging. They pass through soft body tissue more readily than through bone, so different amounts reach the detector. This contrast produces an image in which bones can be distinguished and fractures can be found. X-rays can also be used in some medical treatments.

Gamma rays: sterilisation

Gamma rays are used to sterilise medical equipment and food because the radiation can kill microorganisms. The waves can act through sealed packaging, so an item can be sterilised without being opened. Gamma radiation is also used in some medical procedures.

Match the property to the use

A useful explanation connects an application to what the wave does:

• radio waves and microwaves can carry encoded information;
• suitable microwaves can pass through the atmosphere to satellites;
• absorbed microwaves and infrared transfer energy and cause heating;
• infrared cameras detect infrared emitted by objects;
• visible or infrared signals can travel along optical fibres;
• ultraviolet can make fluorescent materials emit visible light;
• different absorption of X-rays by bone and soft tissue creates image contrast;
• gamma rays can kill microorganisms during sterilisation.