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AQA PhysicsWaves

Electromagnetic spectrum

Order electromagnetic waves and link them to uses and risks.

Start here

The key idea

All electromagnetic waves travel at the same speed in a vacuum but have different frequencies and wavelengths.

Electromagnetic Spectrum
radiomicroIRvisibleUVX-raygammafrequency and energy increase

Use the labels to explain the scientific relationship shown.

Revision notes

The bit that matters

Short notes first. Learn the idea, then use the worked example and questions to check it properly.

1

The electromagnetic spectrum

Electromagnetic (EM) waves are transverse waves that all travel through a vacuum at the same speed, 3×1083 \times 10^{8} m/s.The spectrum, in order of increasing frequency and decreasing wavelength, is: radio, microwave, infrared, visible light, ultraviolet, X-rays and gamma rays.Higher frequency waves carry more energy.EM waves form a continuous spectrum, with visible light being only a small part of it.

2

Uses of EM waves

Radio waves are used for broadcasting and communications; microwaves for cooking and satellite communication; infrared for cooking, heating, remote controls and thermal imaging.Visible light is used in fibre-optic communication and photography; ultraviolet for security marking and producing vitamin D; X-rays for medical imaging of bones; gamma rays for sterilising equipment and treating cancer.The use of each wave depends on its frequency, energy and how it interacts with matter.

3

Dangers and how EM waves are produced

Ultraviolet, X-rays and gamma rays are ionising — they carry enough energy to remove electrons from atoms, which can damage cells and cause cancer or mutations.Radiation dose, measured in sieverts (Sv), describes the risk of harm.Changes in atoms and their nuclei can produce and absorb EM radiation over a wide frequency range; for example, gamma rays come from changes in the nucleus, while visible light and UV come from electron energy level changes.

4

Refraction and emission

Different substances may absorb, transmit, refract or reflect EM waves in ways that vary with wavelength.Refraction at a boundary is caused by the change in wave speed as the wave enters a different medium.A perfect black body is an object that absorbs all radiation incident on it and is also the best possible emitter.The intensity and wavelength distribution of the radiation a body emits depends on its temperature.

Key terms

Definitions to learn

Electromagnetic spectrum

The continuous range of EM waves from radio waves to gamma rays.

Ionising radiation

Radiation with enough energy to remove electrons from atoms (UV, X-rays, gamma).

Refraction

Change in direction of a wave caused by a change in speed at a boundary.

Black body

An object that absorbs all incident radiation and is the best emitter.

Radiation dose

A measure of the risk of harm from ionising radiation, in sieverts (Sv).

Worked example

Put radio waves, visible light and gamma rays in order of increasing frequency.

1

Recall that frequency rises from radio waves towards gamma rays.

2

Place visible light between radio waves and gamma rays.

Final answer

Radio waves, visible light, gamma rays

Exam habit

g. penetration for X-rays, short wavelength for microwaves).Ordering questions: increasing frequency = decreasing wavelength.Learn the sequence radio → microwave → IR → visible → UV → X-ray → gamma.

Watch out

Do not say higher frequency always means safer radiation.

Examiner tips

How to score full marks

  • 1Learn the spectrum order both ways; radio = longest wavelength/lowest frequency, gamma = shortest/highest.
  • 2All EM waves travel at the same speed in a vacuum (3×1083 \times 10^{8} m/s) — a common required fact.
  • 3Link danger to ionising ability: the three highest-frequency waves (UV, X-ray, gamma) are ionising.
Practice questions

Try these yourself

Start with the core skill, then open the answer only after you have attempted the full question.

1Explain why microwaves are used for satellite communication.
Mark scheme
  1. 1.Consider passage through the atmosphere.
Microwaves can pass through the atmosphere and carry information.
2State one risk associated with ultraviolet radiation.
Mark scheme
  1. 1.Link the radiation to living tissue.
Ultraviolet can damage skin cells and increase the risk of skin cancer.
3Explain why X-rays can image bones.
Mark scheme
  1. 1.Compare absorption by bone and soft tissue.
X-rays pass through soft tissue more easily than bone, creating contrast on a detector.
4List the seven groups of the electromagnetic spectrum in order of increasing frequency.[2 marks]
Mark scheme
  1. 1.Recall order from radio to gamma.
Radio, microwave, infrared, visible light, ultraviolet, X-rays, gamma rays (1 for correct full order, 1 for all seven named).
5State one use of microwaves and one use of infrared radiation.[2 marks]
Mark scheme
  1. 1.Give microwave use.
  2. 2.Give infrared use.
Microwaves: cooking food / satellite communication (1). Infrared: cooking/heating / remote controls / thermal imaging (1).
6Explain why X-rays and gamma rays are dangerous to humans.[2 marks]
Mark scheme
  1. 1.They are ionising.
  2. 2.They damage cells.
  3. 3.May cause mutation/cancer.
They are ionising, carrying enough energy to remove electrons from atoms (1), which can damage or kill cells and cause mutations or cancer (1).
7A microwave has a wavelength of 0.12 m. Calculate its frequency. (speed = 3×1083 \times 10^{8} m/s)[3 marks]
Mark scheme
  1. 1.Use f = v / lambda.
  2. 2.Substitute.
  3. 3.Evaluate.
f = v / lambda = (3×1083 \times 10^{8}) / 0.12 (1) = 2.5×1092.5 \times 10^{9} (1) Hz (1).
8Gamma rays and visible light are both EM waves but are produced in different ways and used differently. Compare how each is produced and give one use of each, linking the use to the wave's properties.[4 marks]
Mark scheme
  1. 1.Gamma from nuclear changes.
  2. 2.Visible from electron energy changes.
  3. 3.Use of gamma + reason.
  4. 4.Use of visible + reason.
Gamma rays are produced by changes in the nucleus of unstable atoms (1), whereas visible light is produced by electrons moving between energy levels in atoms (1). Gamma rays are used to sterilise equipment or treat cancer because they are highly ionising and penetrate deeply (1). Visible light is used in fibre-optic communication because it can be transmitted along glass with little absorption (1).
9An X-ray machine emits radiation of wavelength 1.0×10−101.0 \times 10^{-10} m. Calculate the frequency of the X-rays. (speed of EM waves = 3×1083 \times 10^{8} m/s)[2 marks]
Mark scheme
  1. 1.Use f = v / lambda.
  2. 2.Substitute v = 3×1083 \times 10^{8} and lambda = 1.0×10−101.0 \times 10^{-10}.
  3. 3.Evaluate.
f = v / lambda = (3×1083 \times 10^{8}) / (1.0×10−101.0 \times 10^{-10}) (1) = 3×10183 \times 10^{18} Hz (1)
10Explain why ultraviolet radiation is used for fluorescent security marking but visible light is not suitable for this purpose.[3 marks]
Mark scheme
  1. 1.UV causes certain substances to fluoresce (absorb UV, emit visible light).
  2. 2.The marking is invisible in ordinary light.
  3. 3.Under a UV lamp the marking glows visibly.
  4. 4.Visible light would not make the marking hidden in normal conditions.
Certain inks absorb ultraviolet radiation and re-emit the energy as visible light, making them glow (fluorescence) (1); in ordinary visible light these inks appear clear, so the security mark is invisible and cannot be easily forged (1); shining a UV lamp on the object makes the marking glow visibly, allowing verification; visible light would simply reflect off the mark making it always visible and easily copied (1)
11Explain why radio waves are used for long-distance communication rather than ultraviolet radiation, referring to their wavelength, penetration through the atmosphere and ionising ability.[4 marks]
Mark scheme
  1. 1.Radio waves have long wavelengths and can diffract around hills and buildings.
  2. 2.Radio waves are non-ionising so safe for widespread use.
  3. 3.UV is strongly absorbed by the atmosphere (ozone layer).
  4. 4.UV is ionising and would be hazardous.
Radio waves have long wavelengths, which means they can diffract around obstacles such as hills and buildings, allowing broadcasts to reach receivers without a line-of-sight path (1); radio waves pass through the atmosphere without significant absorption, so they can travel long distances (1); ultraviolet radiation is largely absorbed by the ozone layer in the upper atmosphere, so it could not be used for ground-based long-distance communication (1); additionally, UV is ionising and would cause health risks if used widely for communication, whereas radio waves are non-ionising (1)
12Describe what is meant by a 'perfect black body' and explain how the Sun can be modelled as one. State how the peak wavelength of emitted radiation changes as the temperature of a body increases.[3 marks]
Mark scheme
  1. 1.Black body: absorbs all incident radiation; best possible emitter.
  2. 2.Sun approximates a black body — very high absorption.
  3. 3.Peak wavelength shifts to shorter values as temperature increases.
  4. 4.Very hot bodies emit more in UV/visible; cooler ones in infrared.
A perfect black body absorbs all the electromagnetic radiation that falls on it and is also the best possible emitter of radiation for its temperature (1); the Sun is a very good approximation of a black body because its surface is very hot and dense and absorbs almost all incoming radiation (1); as the temperature of a black body increases, it emits more radiation overall and the peak wavelength shifts to shorter (higher frequency) wavelengths — this is why the Sun's peak emission is in the visible range while cooler objects such as the Earth emit mainly infrared (1)
13Evaluate the evidence for and against the claim that mobile phone signals (microwaves) are dangerous to health. In your answer consider the type of radiation, its energy, and how scientists assess risk.[4 marks]
Mark scheme
  1. 1.Microwaves are non-ionising EM waves.
  2. 2.They can cause heating of tissue at high intensities.
  3. 3.Mobile phone signals are at low power — heating effect is negligible.
  4. 4.No conclusive evidence of cancer link from large studies.
  5. 5.Scientific method: correlational studies vs controlled trials; precautionary principle.
Microwaves are non-ionising; they do not carry enough energy per photon to remove electrons from atoms or directly break chemical bonds in DNA, which is the mechanism by which ionising radiation like X-rays causes cancer (1); at high intensities microwaves can heat tissue (as in a microwave oven), but mobile phone signals are many thousands of times less intense, so significant tissue heating is considered negligible (1); large-scale epidemiological studies involving hundreds of thousands of people have so far found no statistically significant increase in cancer rates linked to normal mobile phone use (1); however, some scientists argue that long-term effects are not yet fully known since mobile phone use has been widespread for only a few decades, and apply the precautionary principle by recommending limited use by children; current scientific consensus is that the risk, if any, is very small (1)
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