Electromagnetic spectrum
The electromagnetic spectrum
The electromagnetic spectrum is the complete range of electromagnetic waves. Its regions can be arranged by frequency or by wavelength.
In order of increasing frequency, the main regions are:
- Radio waves
- Microwaves
- Infrared
- Visible light
- Ultraviolet
- X-rays
- Gamma rays
The order of increasing wavelength is the reverse: gamma rays have the shortest wavelengths, while radio waves have the longest.
Frequency, wavelength and speed
All electromagnetic waves travel at the same speed in a vacuum: approximately $$3.0 \times 10^8\ \text{m/s}$$. Their speed, frequency and wavelength are related by:
Here, $$c$$ is wave speed in metres per second, $$f$$ is frequency in hertz, and $$\lambda$$ is wavelength in metres.
In a vacuum, $$c$$ is the same for every electromagnetic wave. Therefore, a higher frequency means a shorter wavelength: if the frequency doubles, the wavelength halves.
Radio waves
Radio waves can carry information from one place to another without a physical connection. They are used to transmit radio and television signals: a transmitter sends information using radio waves, and a receiver detects the waves and recovers the information as sound or pictures.
In radio astronomy, radio telescopes receive radio waves arriving from objects in space. Astronomers study these waves to investigate the objects that produced them. Unlike a radio or television signal, the waves are not sent deliberately to communicate with a receiver.
Radio frequency identification (RFID) uses radio waves to exchange information with tags. An RFID reader sends a radio signal to a tag, and the tag responds with information that identifies it. An object carrying the tag can therefore be identified without direct contact with the reader.
Microwaves
Microwaves can carry information as signals. In mobile phone communication, signals are transmitted using microwaves between a phone and the communication network.
Microwaves can also transfer energy to materials that absorb them. In a microwave oven, energy transferred from the microwaves to the food raises the food’s temperature.
The same heating effect can be harmful to the body. Excessive exposure to microwaves can transfer energy to body cells, heating them internally and causing damage.
Infrared radiation
Infrared radiation is an invisible form of electromagnetic radiation. It transfers energy from a source to an object that absorbs it. This energy transfer can heat the object.
Infrared heating
An electric grill emits infrared radiation from its hot heating element. Food absorbs the radiation, transferring energy to the food and heating it. Human skin can also absorb infrared radiation; excessive exposure can cause burns.
Infrared signals
Infrared radiation can carry information. A television remote controller sends coded infrared signals to a receiver in the television. This is an example of short-range communication.
Optical fibres can also carry information using infrared signals. The signals travel through the fibre to a receiver, which interprets the information they carry.
Infrared detection
Objects emit infrared radiation. A thermal imaging device detects this radiation and uses it to produce an image related to the temperatures of the objects’ surfaces. Warmer and cooler regions can be shown in different colours or shades.
Infrared radiation is also used in intruder alarms. For example, an alarm can detect a change in the infrared radiation reaching a sensor when a person moves into the area being monitored.
Visible light
Visible light is the region of the electromagnetic spectrum that the human eye can detect. We see an object when visible light from it enters our eyes. The light may come directly from a source, or it may be reflected from the object.
Visible light in illumination and photography
In illumination, a light source shines on a scene. Light reflected from objects in the scene enters our eyes, making the objects visible.
In photography, light from a scene enters a camera and is recorded by a sensor or light-sensitive film. This recorded light is used to produce an image of the scene.
Ultraviolet radiation
Ultraviolet (UV) radiation is used to reveal security marks and to sterilise water. It can also damage living cells, so excessive exposure is harmful.
Ultraviolet security marking
Some security marks are not easily visible under ordinary light but become visible when exposed to UV radiation. Marks on banknotes can be checked under a UV lamp to help identify fake notes. UV-visible marks can also be used to identify marked property.
Ultraviolet sterilisation of water
UV radiation can sterilise water by damaging microorganisms in it. The damaged microorganisms can no longer reproduce effectively, reducing the risk that they will cause illness.
Harmful effects of ultraviolet exposure
Excessive UV exposure can damage cells at the surface of the body, including skin cells and cells in the eyes. Damage to skin cells can lead to skin cancer, while damage to the eyes can cause eye conditions.
X-rays
X-rays are a type of radiation used to form images in medical scans and security scanners. They can pass through objects, but different materials transmit different amounts of the radiation.
How X-ray images form
An X-ray scanner directs radiation through the body or an object towards a detector. Some of the radiation is absorbed along the way, while the rest reaches the detector. Areas that transmit different amounts produce different signals, forming a contrasting image.
In a medical scan, bone transmits less X-ray radiation than the surrounding soft tissue, so the two can be distinguished in the image. In a security scanner, differences in transmission help reveal the shapes and materials of objects inside a bag.
Risks of X-ray exposure
Excessive exposure to X-rays can damage cells or cause mutations, which are changes to the genetic material inside cells.
Gamma rays
Gamma rays can damage living cells. This makes them useful for destroying harmful microorganisms and cancer cells, but exposure can also harm healthy cells.
Sterilisation with gamma rays
Gamma radiation is used to sterilise food and medical equipment. It damages or destroys microorganisms, such as bacteria, reducing the risk of infection or food spoilage. The treated food or equipment does not become radioactive simply because it has been exposed to gamma radiation.
Gamma rays in medicine
Gamma radiation can help doctors locate cancer. A small amount of a radioactive substance is introduced into the body, and a detector outside the body detects the gamma radiation it emits. Some substances collect more in certain cancerous tissues, so the detected radiation can help identify where a tumour may be.
Gamma radiation can also be directed at a tumour as a treatment. It damages the cancer cells, including their DNA, so the cells may die or be unable to continue dividing.
Risks of gamma radiation
Gamma radiation can also damage healthy cells. Excessive exposure may kill these cells or damage their DNA, causing mutations. Some mutations can lead to cancer, so exposure must be carefully controlled, including during treatment.
Satellite communication using microwaves
Artificial satellites communicate with Earth mainly using microwaves. These electromagnetic waves carry information as signals and can travel through the atmosphere and through space. A signal sent from Earth can be received by a satellite and transmitted back to Earth.
Some satellite phones communicate through low-orbit satellites. These satellites move across the sky relative to a person on Earth, so a phone may communicate with different satellites as they pass overhead.
Other satellite phones use geostationary satellites. A geostationary satellite orbits Earth so that it remains above the same point on Earth’s surface. Direct broadcast satellite television also uses geostationary satellites: a satellite transmits television signals by microwave to receiving dishes on Earth.
Wireless communication using radio waves
Wireless communication carries information from one device to another using electromagnetic waves travelling through the air. A transmitting aerial sends out the waves, and a receiving aerial detects them. In air, electromagnetic waves travel at approximately the same speed as in a vacuum: about $$3.0 \times 10^8,\mathrm{m/s}$$.
Microwaves and wireless communication
Mobile phones and wireless internet use microwaves. Microwaves can penetrate some walls, allowing devices to communicate without a clear path between them. Their wavelengths are short enough for the transmitting and receiving aerials to be short.
Microwaves are a region of the broader radio-wave spectrum. Bluetooth also uses radio waves to connect devices wirelessly. Its signals can pass through walls, but they are weakened as they do so.
Optical fibre communication
An optical fibre carries information using signals made from visible light or infrared radiation. Changes in the signal represent the information being sent, which can be recovered at the other end of the fibre.
Glass is transparent to visible light and to some infrared radiation, so these signals can travel through a glass fibre. Visible light and short-wavelength infrared can carry data at high rates: a large amount of information can be transmitted each second.
This high data capacity makes optical fibres useful for cable television and high-speed broadband.
Analogue and digital signalling
Sound can be transmitted by converting it into a signal that carries information from one place to another. The signal can represent the sound in either analogue or digital form.
Analogue and digital representations
An analogue signal varies continuously. Changes in the sound are represented by corresponding continuous changes in the signal. For example, as the sound changes, the signal level changes with it and can take any value within its range.
A digital signal represents information using discrete values rather than a continuously varying range of values. These values are commonly binary digits, 0 and 1, represented by distinct signal levels. Sound can be converted into digital information and transmitted using these levels.
Digital signalling and regeneration
Digital signalling can support higher data transmission rates. It also allows a received signal to be regenerated: equipment identifies which discrete value each received level represents, then produces a fresh signal with the corresponding levels.
During transmission, unwanted changes called noise can alter a signal. Regeneration prevents noise from simply accumulating through successive stages, because each fresh signal is produced from the identified values rather than copied with its noise. This can increase the transmission range.
Regeneration works correctly only if the received levels remain distinguishable. If noise changes a level so much that it is identified as the wrong value, the regenerated signal will also contain that error.