Electromagnetic spectrum

The electromagnetic (EM) spectrum is a family of waves that share several key properties. They are all transverse waves, meaning their oscillations are at right angles (perpendicular) to the direction of energy transfer. Unlike sound waves, they do not need a medium to travel and can pass through a vacuum, like outer space. In a vacuum, all EM waves travel at the same incredible speed: the speed of light, approximately 300,000,000 metres per second (3 x 10⁸ m/s). These waves are created by oscillating electric charges and are made of vibrating electric and magnetic fields. A key function of all EM waves is to transfer energy from a source to an absorber.

The electromagnetic spectrum is the complete range of EM waves, ordered by their frequency and wavelength. It is a continuous spectrum, with no gaps. The relationship between frequency and wavelength is inverse: as frequency increases, wavelength decreases. The order of the spectrum, from longest wavelength and lowest frequency to shortest wavelength and highest frequency, is: Radio waves, Microwaves, Infrared, Visible light, Ultraviolet, X-rays, Gamma rays. A helpful mnemonic to remember this order is 'Raging Martians Invaded Venus Using X-ray Guns'.

Radio waves have the longest wavelengths in the EM spectrum and are used primarily for broadcasting and communication. Different frequency bands are used for different purposes, such as AM and FM radio, television broadcasts, and amateur radio. Their long wavelength allows them to diffract (bend) around large objects like hills, making them suitable for terrestrial broadcasting. Microwaves have shorter wavelengths than radio waves. They are used for mobile phone communications, Wi-Fi, and satellite television because they can pass through the Earth's atmosphere without being reflected. In a microwave oven, microwaves are used to heat food. They are absorbed by water molecules in the food, causing the molecules to vibrate rapidly and generate thermal energy.

Infrared (IR) radiation is the region of the EM spectrum just beyond red light. All objects with a temperature above absolute zero emit infrared radiation; the hotter the object, the more IR it emits. This property is used in thermal imaging cameras to 'see' in the dark. Other uses include remote controls for televisions, radiant heaters, and data transmission over short distances (e.g., in some older phones). Visible light is the only part of the EM spectrum that our eyes can detect. It consists of a spectrum of colours, which can be remembered by the mnemonic ROYGBIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet), with red having the longest wavelength and violet the shortest. Visible light is crucial for photography and is also used for communication in optical fibres.

These three types of EM waves have the highest frequencies and energies in the spectrum. This high energy allows them to be ionising, meaning they can knock electrons out of atoms, which can damage or kill living cells. Ultraviolet (UV) light from the sun causes tanning but overexposure can lead to skin cancer and eye damage. It is also used in security marking (fluorescence) and for sterilising water. X-rays have even higher energy and can pass through soft tissues but are absorbed by denser materials like bone, making them ideal for medical imaging. Gamma rays have the highest energy. They are emitted by radioactive materials and are highly penetrating. They are used to sterilise medical equipment and food (as they kill all microbes) and in radiotherapy to destroy cancer cells. Due to their dangers, exposure to X-rays and gamma rays must be minimised, and lead shielding is often used for protection.

Different EM waves are chosen for specific communication tasks based on their properties. Radio waves are used for terrestrial broadcasting (e.g. FM radio) because their long wavelengths can diffract around obstacles. Microwaves are used for satellite communication and mobile phones. They can pass through the Earth's ionosphere to reach satellites and their high frequency allows them to carry a large amount of information (high bandwidth). Mobile phones use microwaves to connect to a local cell tower. Infrared and visible light are used in optical fibres for high-speed broadband internet. Light pulses travel down the thin glass fibres, reflecting off the inside walls via a process called total internal reflection. This method is very secure and can carry huge amounts of data with minimal signal loss.