Full chapter

Electromagnetic spectrum

All 3 topics and the revision summary on one page.

Spectrum order and shared properties

Radio waves, visible light and gamma rays are parts of one electromagnetic spectrum. They differ in wavelength and frequency, while sharing the same speed in vacuum.

Wavelength is a distance between equivalent adjacent points of a wave. Frequency is the number of complete cycles per second, in hertz. A wave transfers energy as it propagates.

What all electromagnetic waves share

All electromagnetic waves are transverse: their oscillations are perpendicular to the direction of propagation. The oscillating quantities are electric and magnetic fields, rather than air particles moving up and down.

They can travel through a vacuum, without a material medium. All travel at the same vacuum speed. For the calculations here, use the supplied value c = 3.0 x 10⁸ m/s.

A sinusoidal drawing can represent how a field varies. It is not a wavy path followed by a photon or a row of air particles carrying the radiation. The vacuum-speed statement also does not say that every real communication system has the same total delay: material paths and equipment can introduce other effects.

Order wavelength and frequency in opposite directions

Read the same order in two directions

Moving down this list, frequency increases and wavelength decreases. All electromagnetic waves have the same speed in vacuum.

Visible light, enlarged

The separate region names are useful categories. Microwaves are also commonly described as part of the wider radio-frequency range.

From radio waves towards gamma rays, frequency increases and wavelength decreases. The region sizes are schematic, not equal numerical bandwidths. The enlarged visible section places red at the longer-wavelength end and violet at the shorter-wavelength end.

Increasing frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.

Increasing wavelength: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio waves.

Within visible light, red has a longer wavelength and lower frequency than violet. Both are electromagnetic waves. The named regions are convenient divisions of a continuous spectrum, not physical gaps; microwaves are also often included within the wider radio-frequency range.

c = fλ so λ = c/fFor a vacuum wave, c is the common speed, f is frequency in Hz and λ is wavelength in m. Higher frequency means shorter wavelength at this fixed speed, not faster travel.

Worked comparison

Two frequencies, one vacuum speed

A 100 MHz signal has f = 100 x 10⁶ = 1.0 x 10⁸ Hz. Its wavelength is λ = (3.0 x 10⁸)/(1.0 x 10⁸) = 3.0 m.

A 2.5 GHz signal has f = 2.5 x 10⁹ Hz. Its wavelength is λ = (3.0 x 10⁸)/(2.5 x 10⁹) = 0.12 m.

The second signal repeats more often and has more closely spaced wave cycles. Both still travel at c in vacuum.

Work from wavelength when that is supplied

One nanometre is 10^-9 m. For a supplied wavelength of 600 nm, λ = 6.00 x 10^-7 m, so f = c/λ = 5.0 x 10¹⁴ Hz.

At 1200 nm, the wavelength is twice as large and the frequency is half as large: 2.5 x 10¹⁴ Hz. Wavelength and frequency cannot both increase while their product remains the same speed.

Radio reception ends with a different wave

A radio transmitter sends an electromagnetic signal to a receiver. The receiver processes the information and drives a loudspeaker. The loudspeaker then makes sound waves in the surrounding air.

The radio signal can cross a vacuum; the sound from the loudspeaker needs a material medium. In air, sound is longitudinal, while the radio wave is transverse. Hearing a radio programme does not mean that the broadcast crossed the space between transmitter and receiver as sound.

Travel-time comparison

Equal paths through vacuum

Two electromagnetic signals are emitted together and each follows a 900 km vacuum path. Convert the distance to 900000 m.

t = distance/speed = 900000/(3.0 x 10⁸) = 0.0030 s = 3.0 ms.

Both arrive together in this ideal comparison, regardless of their different frequencies. This is the propagation time for the stated paths, not the complete delay of a working network.

Keep frequency separate from speed. More cycles per second does not mean faster propagation when the vacuum speed is fixed.

Optional check A 100 MHz signal and a 2.5 GHz signal start together and follow equal 900 km paths through vacuum. Which comparison is correct?

Uses across the spectrum

To explain an application, identify the source, what happens to the radiation, and what is detected or changed.

All seven regions are electromagnetic waves. An application may use transmission to carry information, absorption to transfer energy, or differences in detection to form an image. Some applications use more than one region.

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.

Identify which device emits and which receives

Green arrows show infrared travelling from a source to a detector. These are schematic device drawings; infrared itself is invisible.

A remote emits a coded infrared signal

The remote actively emits. The receiver detects the changing signal and the appliance responds.

A passive intruder sensor receives infrared

A person and the surroundings emit infrared. Motion changes the pattern received by this sensor; it does not send out its own detecting beam.

The remote is the source of a coded infrared signal received by the appliance. The passive intruder sensor receives changing infrared from its view; it is not the source of a detecting beam.

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.

Two imaging methods: locate the source first

The source position changes what is detected

Arrows show radiation travelling towards or away from the object. Both panels are schematics, not clinical images.

X-ray transmission: source outside the object

The darker patch inside the object is more absorbing. Its fainter transmitted arrow indicates less radiation reaching the detector; no numerical intensity scale is implied.

Gamma-tracer detection: source inside the body

Radiation that escapes from an appropriate tracer can reveal where the tracer is distributed. This detection purpose differs from directing radiation at tissue for treatment.

The X-ray transmission image uses an external source and records radiation passing through the object. The gamma-tracer example detects radiation emitted from within it. Both need detectors, but the source locations and the information obtained differ.

Worked comparison

Transmission image or tracer image?

In the X-ray example, the source is outside the body and the detector is on the far side. The image depends on how much radiation travels through different paths. Differences in absorption and transmission help reveal structure.

In the gamma-tracer example, radiation originates from a suitable tracer distributed inside the body. A detector outside records the emissions and helps build an image of that distribution. This is different from passing a beam through the body from an external source.

Detecting cancer and treating cancer are also different tasks. A tracer image can provide information about tissue, while treatment deliberately uses radiation to damage targeted cells.

Invisible does not mean undetectable

Suppose a suitable detector responds just beyond the red end of a spread-out visible spectrum, where the eye sees no light. Blocking the incoming beam reduces that response. This supports the presence of invisible radiation reaching the detector.

A detector that fails to respond in another region may simply be insensitive there. Its non-response does not prove that no radiation is present. A thermal camera, ordinary camera and radio receiver are designed to respond to different parts of the spectrum.

A use is not an exclusive label. Optical fibres can carry visible or infrared signals, and medical imaging can involve different regions and mechanisms. Explain the particular source, transfer and detector in the question.

Optional check An infrared remote operates a receiver, while a passive infrared intruder sensor detects a moving warm body. Which account correctly identifies the radiation sources?

Exposure effects

Absorbed electromagnetic radiation transfers energy to matter. Over-exposure can damage living cells and tissue through heating or changes to atoms and molecules.

Thermal radiation can transfer energy without a material medium. The effect on a body depends on what it absorbs, not just on whether radiation is present.

Heating: absorbed energy raises temperature

Absorbed microwave or infrared radiation can increase a material's internal energy and temperature. If tissue gains energy faster than it can transfer that energy away, its temperature can rise enough to cause damage, including burns.

The microwave oven uses absorption for useful heating. That application does not imply that every microwave signal has the same heating effect: the power reaching an object, the amount absorbed, the exposure duration and the conditions differ.

Heating explanation

Radiation is absorbed → internal energy increases → temperature can rise → excessive heating damages tissue.

This is an energy-transfer explanation. Microwaves do not need to make matter radioactive in order to heat it, and non-ionising radiation is not the same as radiation that cannot cause harm.

Ionisation: electrons are removed

Ionisation is the removal of electrons from atoms or molecules, producing ions. X-rays and gamma rays provide clear examples of ionising radiation. Their interactions can damage molecules in cells, including DNA.

Ionising explanation

Radiation removes electrons → ions and molecular changes are produced → cells can be damaged.

Ionisation is a different microscopic mechanism from simply making tissue hotter. Radiation can cause molecular damage without a large noticeable temperature rise. The fact that a beam can be used to destroy cancer cells also explains why exposure of healthy tissue matters.

Ultraviolet needs a careful distinction

Some sufficiently energetic ultraviolet radiation can ionise, but not all UV is ionising. UV can also damage cells through other changes in molecules. Its hazards should not be explained by claiming that every UV wave removes electrons.

UV can damage skin and eyes, and UV exposure from sunbeds increases skin-cancer risk. A useful application, such as disinfecting water, does not make unintended exposure harmless.

Apply the same mechanism

Disinfecting water in a screened unit

The unit exposes microorganisms in the water to UV that can damage their genetic material and prevent reproduction. The intended effect is useful because it reduces viable microorganisms under the treatment conditions.

If that radiation reaches a person's tissue unintentionally, the ability to affect living cells has not disappeared. The same application therefore needs the radiation to reach its intended target while exposure elsewhere is controlled.

The explanation does not require every UV interaction to be ionisation. Identify the cellular damage and the exposure conditions instead of using "non-ionising" as a synonym for harmless.

Use exposure information, not a single spectrum label

To compare effects, consider the radiation type, energy reaching and being absorbed by the tissue, duration and the tissue exposed. Wavelength or frequency alone does not determine the outcome of every exposure.

The spectrum is therefore not divided into a universally safe side and a universally harmful side. Heating and ionisation explain important hazards, while the amount absorbed and the conditions determine how those mechanisms affect a particular case.

Name the mechanism. For a heating effect, explain absorption and temperature rise. For an ionising effect, explain electron removal and possible cellular damage. For UV, retain the qualification that harm is not always caused by ionisation.

Optional check A screened ultraviolet water-disinfection unit uses radiation that can damage microorganisms. Why does that useful effect not establish that unintended exposure of a person is harmless?

Revision summary

One spectrum, two orderings

Increasing frequency, decreasing wavelength: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.

Increasing wavelength, decreasing frequency: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio waves. Within visible light, red is at the longer-wavelength end and violet at the shorter-wavelength end.

Shared properties: All electromagnetic waves are transverse and travel at the same speed in vacuum. They can travel without a material medium; sound cannot.
Frequency and wavelength: c = fλ; λ = c/f; f = c/λ. For the examples here, c = 3.0 x 10⁸ m/s. Higher frequency means shorter wavelength at the same speed.
Prefix reminders: 1 MHz = 10⁶ Hz; 1 GHz = 10⁹ Hz; 1 nm = 10^-9 m. Convert to compatible units before substitution.

Applications to recall and explain

Radio waves: Radio and television communication; Astronomy; RFID tags.
Microwaves: Mobile phones; Microwave ovens; Satellite television.
Infrared: Infrared remote controllers; Intruder alarms; Thermal imaging.
Visible light: Photography; Optical fibres in medicine; Optical fibres in telecommunications.
Ultraviolet: Sunbeds; Bank-note authentication; Disinfecting water.
X-rays: Medical radiology; Security screening; Industrial defect detection.
Gamma rays: Sterilising food; Detecting cancer; Treating cancer.

For any application, identify the source, the radiation's path or absorption, and the detector or intended effect. A passive infrared alarm receives radiation; a remote emits a coded signal. An X-ray transmission image uses an external source; a gamma-tracer image records emissions from a tracer inside the body. Some optical-fibre links use visible light, while many use infrared.

Heating and ionising effects

Heating: absorption raises internal energy and can raise temperature. Excessive heating can damage tissue.
Ionisation: radiation removes electrons from atoms or molecules. The resulting changes can damage cells and DNA. X-rays and gamma rays are ionising examples.
Ultraviolet: some sufficiently energetic UV is ionising, but not all UV is. UV can harm skin and eyes through molecular damage; sunbed exposure increases skin-cancer risk.
Exposure conditions: the amount absorbed, duration, radiation type and tissue exposed matter. An application is not evidence that every exposure is harmless.

Keep the radiation distinct from what a device produces or displays: a radio receiver makes sound through a loudspeaker, and a thermal image uses assigned display colours for detected infrared information.

Back to spectrum order