The electromagnetic spectrum

What you'll learn

Why electromagnetic waves are transverse waves that can travel through space.
How wavelength and frequency change across the electromagnetic spectrum.
The main regions of the spectrum, from radio waves to gamma rays.
How different electromagnetic waves are used, including in communication, heating and medical imaging.

Starting point: waves transfer energy

You already know that a wave is a way of transferring energy from one place to another without transferring matter overall. For example, sound can transfer energy through air, and water waves can transfer energy across the surface of water.

Electromagnetic waves are another huge family of waves. They include visible light, radio waves, microwaves, infrared, ultraviolet, X-rays and gamma rays.

Definition

Key wave quantities

Wavelength, symbol $λ\lambda$ , is the distance from one point on a wave to the matching point on the next wave, such as crest to crest. It is measured in metres (m).
Frequency, symbol $f$ , is the number of waves passing a point each second. It is measured in hertz (Hz).
Wave speed, symbol $v$ , is how fast the wave energy travels. It is measured in metres per second (m/s).

Electromagnetic waves are transverse

A transverse wave is a wave where the vibrations are at right angles to the direction the wave travels. Electromagnetic waves are transverse.

Unlike sound waves, electromagnetic waves do not need particles to travel through. This means they can travel through a vacuum, such as space.

In a vacuum, all electromagnetic waves travel at the same speed:

\approx 3.0 \times 10^8\ \text{m/s}

This is often called the speed of light, because visible light is one type of electromagnetic wave.

Key Idea

Same wave family

Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays are all electromagnetic waves. They differ in wavelength and frequency, but in a vacuum they all travel at the same speed.

Common Mistake

Higher frequency does not mean faster in space

In a vacuum, gamma rays do not travel faster than radio waves. Gamma rays have a higher frequency and shorter wavelength, but the wave speed is the same.

Energy transfer: source to absorber

Electromagnetic waves transfer energy from a source to an absorber. A source is where the wave is emitted from. An absorber is the object or material that takes in the wave’s energy.

Examples include:

The Sun emits visible light, infrared and ultraviolet; your skin absorbs some of this energy.
A microwave oven emits microwaves; food absorbs energy and heats up.
A radio transmitter emits radio waves; a receiving aerial absorbs some energy and produces an electrical signal.
A gamma source can transfer energy to cancer cells in radiotherapy.

Example

Tracing energy transfer in a microwave oven

Identify the source: the microwave oven produces microwaves using an electrical circuit inside the oven.
Identify the absorber: water molecules and other particles in the food absorb some microwave energy, while much of the container may transmit it.
Link absorption to the effect: the absorbed energy increases the internal energy of the food, so its temperature rises.

Frequency and wavelength are linked

For any wave, the wave speed equation is:

f\lambda

For electromagnetic waves in a vacuum, $v$ is the same for all parts of the spectrum. So if frequency increases, wavelength must decrease.

That is why the spectrum goes from long wavelength, low frequency radio waves to short wavelength, high frequency gamma rays.

Example

Calculating the frequency of a microwave

A microwave in air has a wavelength of 0.12 m. Estimate its frequency, using $\times 10^8\ \text{m/s}$ .

Rearrange the wave equation to make frequency the subject:
$\frac{v}{\lambda}$
Substitute the wave speed and wavelength:
$\frac{3.0 \times 10^8\ \text{m/s}}{0.12\ \text{m}}$
Calculate and give the unit:
$\times 10^9\ \text{Hz}$
This is a very high frequency, which fits with microwaves being beyond radio waves in the spectrum.

Tip

Microwave practical link

In some practicals, melted spots in chocolate or processed cheese can be used to estimate microwave wavelength. Once you know $λ\lambda$ , you can use $f\lambda$ to connect wavelength and frequency.

The order of the electromagnetic spectrum

The main groupings of the electromagnetic spectrum, from longest wavelength to shortest wavelength, are:

Radio waves
Microwaves
Infrared
Visible light: red, orange, yellow, green, blue, violet
Ultraviolet
X-rays
Gamma rays

The electromagnetic spectrum is continuous. The names are useful groupings, not completely separate “things”.

Diagram of the electromagnetic spectrum from radio waves to gamma rays

Tip

Remembering the order

A common mnemonic is: Raging Martians Invaded Venus Using X-ray Guns — Radio, Microwave, Infrared, Visible, Ultraviolet, X-rays, Gamma.

Your eyes only detect a small part

Your eyes can only detect visible light, which is a small region of the electromagnetic spectrum.

You cannot directly see infrared from a TV remote, ultraviolet from the Sun, or X-rays from an X-ray machine. Instead, detectors convert these waves into electrical signals or visible images.

Visible light itself runs from red to violet:

Red light has the longest wavelength in the visible region.
Violet light has the shortest wavelength in the visible region.

Uses of electromagnetic waves

Different electromagnetic waves are useful because they have different wavelengths, frequencies, absorption behaviours and penetrating abilities.

Region	Some practical uses	Why it is useful
Radio waves	Radio and TV broadcasting	Can travel long distances and can be produced and detected by aerials
Microwaves	Mobile phone signals, satellite communication, microwave cooking	Can carry information; can be absorbed by water in food
Infrared	Heaters, thermal imaging cameras, remote controls	Emitted by warm objects; can transfer thermal energy
Visible light	Seeing, photography, optical fibres, endoscopes	Detected by eyes and cameras; can carry images
Ultraviolet	Security marking, fluorescent lamps, sterilising surfaces or water	Can cause some materials to fluoresce; can kill microorganisms
X-rays	Medical imaging, airport security scanners	Can pass through soft tissue but are absorbed more by dense materials like bone
Gamma rays	Sterilising medical equipment, cancer radiotherapy, radioactive tracers	Very penetrating; can kill cells or be detected from outside the body

Hazards: ultraviolet, X-rays and gamma rays

Some electromagnetic waves can be hazardous to human body tissues.

Ultraviolet can damage skin cells and eyes. It can cause sunburn and increase the risk of skin cancer.

X-rays and gamma rays are ionising radiation. This means they can damage atoms and molecules in cells, including DNA. This can kill cells or increase the risk of cancer.

Key Idea

Risk depends on absorption and dose

A wave becomes hazardous when body tissue absorbs enough energy to cause damage. In medicine, the benefit of using radiation must be balanced against the risk from the dose.

Common Mistake

Useful does not mean harmless

X-rays and gamma rays are very useful in medicine, but they can still be dangerous. Safety measures reduce exposure time, increase distance where possible, and use shielding such as lead screens.

Medical imaging and hidden structures

This next idea is separate Physics only for J249, not Combined Science.

Waves can help us detect structures hidden from direct observation because different materials absorb, transmit and reflect waves differently. In the body, waves may behave differently in solids and liquids, or at boundaries between tissues.

Absorption means the wave’s energy is taken in by a material.
Reflection means the wave bounces back from a boundary.
Transmission means the wave passes through a material.
A change in velocity between materials can help cause reflection at a boundary.

The diagram shows how different waves can reveal hidden structures in the body.

Schematic comparing X-ray imaging, gamma tracer imaging and ultrasound echoes

Examples in medical imaging

Infrared imaging detects infrared emitted by the skin. Warmer areas emit more infrared, so it can show temperature patterns, such as inflammation or poor circulation near the surface.

X-ray imaging works because X-rays pass through soft tissue more easily than through bone. Bone absorbs more X-rays, so it appears as a darker or lighter shadow depending on the detector display.

Gamma imaging often uses a radioactive tracer inside the body. The tracer emits gamma rays, which pass out of the body and are detected. This can show how an organ is functioning, not just what it looks like.

Ultrasound is not electromagnetic. It is a mechanical wave. It is used as an alternative because echoes reflect from boundaries between tissues. The time taken for echoes to return helps build an image.

Example

Choosing a medical imaging method

A doctor needs to image a suspected broken wrist and also check a developing foetus.

For the wrist, compare absorption: bone absorbs X-rays much more than surrounding soft tissue, so an X-ray image gives strong contrast for a fracture.
For the foetus, compare risk: X-rays and gamma rays are ionising, so ultrasound is usually preferred because it does not use ionising radiation.
Link ultrasound to imaging: ultrasound waves reflect from boundaries between tissues, so returning echoes can be used to build up an image.

Radio waves and electrical circuits

An oscillation is a repeated back-and-forth change. In an electrical circuit, this can mean charges moving back and forth, creating an alternating current.

Radio waves can be:

Produced by oscillations in electrical circuits. For example, an alternating current in a transmitting aerial produces radio waves.
Used to induce oscillations in electrical circuits. For example, incoming radio waves make electrons oscillate in a receiving aerial, producing a tiny electrical signal.

Key Idea

Radio aerials work both ways

A transmitting aerial turns electrical oscillations into radio waves. A receiving aerial turns radio waves back into electrical oscillations.

Exam technique

In the exam

If asked for the spectrum order, also mention the direction: radio has the longest wavelength and lowest frequency; gamma has the shortest wavelength and highest frequency.
For calculations, use $f\lambda$ and remember that electromagnetic waves in a vacuum travel at about $3.0 \times 10^8\ \text{m/s}$ .
For uses and dangers, link your answer to physics words such as absorption, reflection, transmission, energy transfer and ionising radiation.

Self review

Check yourself

What happens to wavelength when the frequency of an electromagnetic wave increases in a vacuum?
Why can X-rays show bones clearly, but ultrasound is often preferred for imaging a foetus?
How can a radio aerial both transmit and receive radio waves?

The electromagnetic spectrum

What you'll learn

Why electromagnetic waves are transverse waves that can travel through space.
How wavelength and frequency change across the electromagnetic spectrum.
The main regions of the spectrum, from radio waves to gamma rays.
How different electromagnetic waves are used, including in communication, heating and medical imaging.

Starting point: waves transfer energy

Electromagnetic waves are another huge family of waves. They include visible light, radio waves, microwaves, infrared, ultraviolet, X-rays and gamma rays.

Definition

Key wave quantities

Wavelength, symbol $λ\lambda$ , is the distance from one point on a wave to the matching point on the next wave, such as crest to crest. It is measured in metres (m).
Frequency, symbol $f$ , is the number of waves passing a point each second. It is measured in hertz (Hz).
Wave speed, symbol $v$ , is how fast the wave energy travels. It is measured in metres per second (m/s).

Electromagnetic waves are transverse

A transverse wave is a wave where the vibrations are at right angles to the direction the wave travels. Electromagnetic waves are transverse.

Unlike sound waves, electromagnetic waves do not need particles to travel through. This means they can travel through a vacuum, such as space.

In a vacuum, all electromagnetic waves travel at the same speed:

\approx 3.0 \times 10^8\ \text{m/s}

This is often called the speed of light, because visible light is one type of electromagnetic wave.

Key Idea

Same wave family

Common Mistake

Higher frequency does not mean faster in space

In a vacuum, gamma rays do not travel faster than radio waves. Gamma rays have a higher frequency and shorter wavelength, but the wave speed is the same.

Energy transfer: source to absorber

Electromagnetic waves transfer energy from a source to an absorber. A source is where the wave is emitted from. An absorber is the object or material that takes in the wave’s energy.

Examples include:

The Sun emits visible light, infrared and ultraviolet; your skin absorbs some of this energy.
A microwave oven emits microwaves; food absorbs energy and heats up.
A radio transmitter emits radio waves; a receiving aerial absorbs some energy and produces an electrical signal.
A gamma source can transfer energy to cancer cells in radiotherapy.

Example

Tracing energy transfer in a microwave oven

Identify the source: the microwave oven produces microwaves using an electrical circuit inside the oven.
Identify the absorber: water molecules and other particles in the food absorb some microwave energy, while much of the container may transmit it.
Link absorption to the effect: the absorbed energy increases the internal energy of the food, so its temperature rises.

Frequency and wavelength are linked

For any wave, the wave speed equation is:

f\lambda

For electromagnetic waves in a vacuum, $v$ is the same for all parts of the spectrum. So if frequency increases, wavelength must decrease.

That is why the spectrum goes from long wavelength, low frequency radio waves to short wavelength, high frequency gamma rays.

Example

Calculating the frequency of a microwave

A microwave in air has a wavelength of 0.12 m. Estimate its frequency, using $\times 10^8\ \text{m/s}$ .

Rearrange the wave equation to make frequency the subject:
$\frac{v}{\lambda}$
Substitute the wave speed and wavelength:
$\frac{3.0 \times 10^8\ \text{m/s}}{0.12\ \text{m}}$
Calculate and give the unit:
$\times 10^9\ \text{Hz}$
This is a very high frequency, which fits with microwaves being beyond radio waves in the spectrum.

Tip

Microwave practical link

In some practicals, melted spots in chocolate or processed cheese can be used to estimate microwave wavelength. Once you know $λ\lambda$ , you can use $f\lambda$ to connect wavelength and frequency.

The order of the electromagnetic spectrum

The main groupings of the electromagnetic spectrum, from longest wavelength to shortest wavelength, are:

Radio waves
Microwaves
Infrared
Visible light: red, orange, yellow, green, blue, violet
Ultraviolet
X-rays
Gamma rays

The electromagnetic spectrum is continuous. The names are useful groupings, not completely separate “things”.

Diagram of the electromagnetic spectrum from radio waves to gamma rays

Tip

Remembering the order

A common mnemonic is: Raging Martians Invaded Venus Using X-ray Guns — Radio, Microwave, Infrared, Visible, Ultraviolet, X-rays, Gamma.

Your eyes only detect a small part

Your eyes can only detect visible light, which is a small region of the electromagnetic spectrum.

You cannot directly see infrared from a TV remote, ultraviolet from the Sun, or X-rays from an X-ray machine. Instead, detectors convert these waves into electrical signals or visible images.

Visible light itself runs from red to violet:

Red light has the longest wavelength in the visible region.
Violet light has the shortest wavelength in the visible region.

Uses of electromagnetic waves

Different electromagnetic waves are useful because they have different wavelengths, frequencies, absorption behaviours and penetrating abilities.

Region	Some practical uses	Why it is useful
Radio waves	Radio and TV broadcasting	Can travel long distances and can be produced and detected by aerials
Microwaves	Mobile phone signals, satellite communication, microwave cooking	Can carry information; can be absorbed by water in food
Infrared	Heaters, thermal imaging cameras, remote controls	Emitted by warm objects; can transfer thermal energy
Visible light	Seeing, photography, optical fibres, endoscopes	Detected by eyes and cameras; can carry images
Ultraviolet	Security marking, fluorescent lamps, sterilising surfaces or water	Can cause some materials to fluoresce; can kill microorganisms
X-rays	Medical imaging, airport security scanners	Can pass through soft tissue but are absorbed more by dense materials like bone
Gamma rays	Sterilising medical equipment, cancer radiotherapy, radioactive tracers	Very penetrating; can kill cells or be detected from outside the body

Hazards: ultraviolet, X-rays and gamma rays

Some electromagnetic waves can be hazardous to human body tissues.

Ultraviolet can damage skin cells and eyes. It can cause sunburn and increase the risk of skin cancer.

X-rays and gamma rays are ionising radiation. This means they can damage atoms and molecules in cells, including DNA. This can kill cells or increase the risk of cancer.

Key Idea

Risk depends on absorption and dose

A wave becomes hazardous when body tissue absorbs enough energy to cause damage. In medicine, the benefit of using radiation must be balanced against the risk from the dose.

Common Mistake

Useful does not mean harmless

X-rays and gamma rays are very useful in medicine, but they can still be dangerous. Safety measures reduce exposure time, increase distance where possible, and use shielding such as lead screens.

Medical imaging and hidden structures

This next idea is separate Physics only for J249, not Combined Science.

Absorption means the wave’s energy is taken in by a material.
Reflection means the wave bounces back from a boundary.
Transmission means the wave passes through a material.
A change in velocity between materials can help cause reflection at a boundary.

The diagram shows how different waves can reveal hidden structures in the body.

Schematic comparing X-ray imaging, gamma tracer imaging and ultrasound echoes

Examples in medical imaging

Infrared imaging detects infrared emitted by the skin. Warmer areas emit more infrared, so it can show temperature patterns, such as inflammation or poor circulation near the surface.

X-ray imaging works because X-rays pass through soft tissue more easily than through bone. Bone absorbs more X-rays, so it appears as a darker or lighter shadow depending on the detector display.

Example

Choosing a medical imaging method

A doctor needs to image a suspected broken wrist and also check a developing foetus.

For the wrist, compare absorption: bone absorbs X-rays much more than surrounding soft tissue, so an X-ray image gives strong contrast for a fracture.
For the foetus, compare risk: X-rays and gamma rays are ionising, so ultrasound is usually preferred because it does not use ionising radiation.
Link ultrasound to imaging: ultrasound waves reflect from boundaries between tissues, so returning echoes can be used to build up an image.

Radio waves and electrical circuits

An oscillation is a repeated back-and-forth change. In an electrical circuit, this can mean charges moving back and forth, creating an alternating current.

Radio waves can be:

Produced by oscillations in electrical circuits. For example, an alternating current in a transmitting aerial produces radio waves.
Used to induce oscillations in electrical circuits. For example, incoming radio waves make electrons oscillate in a receiving aerial, producing a tiny electrical signal.

Key Idea

Radio aerials work both ways

A transmitting aerial turns electrical oscillations into radio waves. A receiving aerial turns radio waves back into electrical oscillations.

Exam technique

In the exam

If asked for the spectrum order, also mention the direction: radio has the longest wavelength and lowest frequency; gamma has the shortest wavelength and highest frequency.
For calculations, use $f\lambda$ and remember that electromagnetic waves in a vacuum travel at about $3.0 \times 10^8\ \text{m/s}$ .
For uses and dangers, link your answer to physics words such as absorption, reflection, transmission, energy transfer and ionising radiation.

Self review

Check yourself

What happens to wavelength when the frequency of an electromagnetic wave increases in a vacuum?
Why can X-rays show bones clearly, but ultrasound is often preferred for imaging a foetus?
How can a radio aerial both transmit and receive radio waves?

The electromagnetic spectrum

What you'll learn

Starting point: waves transfer energy

Electromagnetic waves are transverse

Energy transfer: source to absorber

Frequency and wavelength are linked

The order of the electromagnetic spectrum

Your eyes only detect a small part

Uses of electromagnetic waves

Hazards: ultraviolet, X-rays and gamma rays

Medical imaging and hidden structures

Examples in medical imaging

Radio waves and electrical circuits

Recap questions

The electromagnetic spectrum

What you'll learn

Starting point: waves transfer energy

Electromagnetic waves are transverse

Energy transfer: source to absorber

Frequency and wavelength are linked

The order of the electromagnetic spectrum

Your eyes only detect a small part

Uses of electromagnetic waves

Hazards: ultraviolet, X-rays and gamma rays

Medical imaging and hidden structures

Examples in medical imaging

Radio waves and electrical circuits

Recap questions

The electromagnetic spectrum

What you'll learn

Starting point: waves transfer energy

Electromagnetic waves are transverse

Energy transfer: source to absorber

Frequency and wavelength are linked

The order of the electromagnetic spectrum

Your eyes only detect a small part

Uses of electromagnetic waves

Hazards: ultraviolet, X-rays and gamma rays

Medical imaging and hidden structures

Examples in medical imaging

Radio waves and electrical circuits

Recap questions

The electromagnetic spectrum Revision Guide

The electromagnetic spectrum

What you'll learn

Starting point: waves transfer energy

Electromagnetic waves are transverse

Energy transfer: source to absorber

Frequency and wavelength are linked

The order of the electromagnetic spectrum

Your eyes only detect a small part

Uses of electromagnetic waves

Hazards: ultraviolet, X-rays and gamma rays

Medical imaging and hidden structures

Examples in medical imaging

Radio waves and electrical circuits

Recap questions

The electromagnetic spectrum Revision Guide