14.1 Common properties

All electromagnetic waves are transverse oscillations of electric and magnetic fields. They can travel through a vacuum and carry energy and information. In a vacuum, every region travels at 3.0 × 10⁸ m/s; in air the speed is approximately the same for O Level calculations.

The regions form one continuous spectrum rather than separate types with sharp natural boundaries. Their different interactions with matter arise mainly from differences in frequency, wavelength and photon energy.

14.2 Order and trends

In order of increasing frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Wavelength decreases in the same direction. Photon energy and ionising ability generally increase toward the high-frequency end.

Use v = fλ for any electromagnetic wave in vacuum or air. Because v is common, a very high frequency corresponds to a very short wavelength. Learn both directions of the order so questions cannot reverse it unexpectedly.

14.3 Radio waves and microwaves

Radio waves are used for radio and television communications and for astronomy. Their long wavelengths can diffract around obstacles, and different frequencies are chosen for different broadcasting and detection tasks. Radio telescopes detect emissions from space that may be invisible to optical telescopes.

Microwaves are used in satellite television, mobile phones, Bluetooth and microwave ovens. Satellite links use microwaves that can pass through the atmosphere and be directed in narrow beams. In ovens, microwaves transfer energy to molecules in food, producing heating; the syllabus does not require a detailed molecular-resonance account.

14.4 Infrared and visible light

Infrared is emitted strongly by warm objects and is used in heaters, thermal imaging, intruder alarms and remote controllers. It is also used in optical-fibre communication, where infrared sources can give low loss in suitable fibres.

Visible light enables vision and photography. Cameras form real images on film or electronic sensors. Visible light can also carry information through fibres or free-space links, but its named syllabus applications are photography and vision.

14.5 Ultraviolet

Ultraviolet can cause fluorescent materials to emit visible light. It is used in security marking and checking counterfeit bank notes because hidden markings fluoresce. Its ability to damage microorganisms is used to sterilise water.

Ultraviolet has higher frequency than visible light and is ionising enough to damage living tissue. Applications must therefore balance effectiveness with controlled exposure.

14.6 X-rays and gamma rays

X-rays penetrate soft tissue more readily than bone, allowing medical imaging. They are also used in security scanners and engineering to detect internal cracks in metal. High doses can kill cancer cells, although gamma radiation is also widely used for this purpose.

Gamma rays are used to detect and kill cancerous cells, sterilise food and medical equipment, and inspect metal structures. Their strong penetration can be useful, but shielding and dose control are essential. Gamma rays originate from nuclear changes, while X-rays are commonly generated by fast electrons; both are electromagnetic radiation.

14.7 Choosing a region

Application questions reward a link between property and use. For example, X-rays penetrate soft tissue but are absorbed more by bone; infrared is detected from warm objects; microwaves can be directed to satellites; gamma rays penetrate packaging and kill microorganisms.

A list of uses without explanations may gain limited credit when the command word is “explain”. State the relevant property, the role it plays and any safety consideration.

Worked examples

Practical focus

Examination guidance

Check your understanding

1. Which region lies between infrared and ultraviolet?
2. State the vacuum speed of electromagnetic waves.
3. Why are microwaves suitable for satellite communication?