Electromagnetic Waves

An electromagnetic wave is a self-propagating oscillation of an electric field and a magnetic field at right angles to each other and to the direction the wave travels. There is no need for a material medium — unlike sound, EM waves can cross a vacuum, which is why sunlight reaches Earth across millions of kilometres of empty space.

All EM waves share the same key properties:

• Transverse. The oscillation (of the fields) is perpendicular to the direction of energy transfer.
• Travel at the same speed in a vacuum. $c = 3 \times 10^8$ m/s — by far the highest speed in physics.
• Transfer energy (and information) from source to absorber.
• Obey the wave equation $v = f\lambda$. As frequency rises, wavelength falls.

EM waves originate from changes in atoms and the nuclei of atoms. Examples in the AQA spec:

• An oscillating electric current in an aerial radiates radio waves.
• Hot objects radiate infrared (and visible light if hot enough).
• Electron transitions in atoms emit visible and ultraviolet.
• Excited inner electrons or rapidly decelerated electrons emit X-rays.
• Changes in the nucleus (radioactive decay) emit gamma rays.

Why this matters for the exam. AQA marks the phrase "transverse waves that transfer energy from source to observer" — keep that exact form of words in mind.

Because EM waves can have any wavelength from kilometres down to a fraction of a femtometre, physicists slice the continuous spectrum into seven named groups purely for convenience. The groups blend smoothly into each other — there is no sharp boundary, for example, between a long microwave and a short radio wave.

In increasing frequency (or equivalently decreasing wavelength), the spec order is:

1. Radio waves — longest wavelength (km down to ~1 m).
2. Microwaves (~30 cm down to ~1 mm).
3. Infrared (~1 mm down to ~700 nm).
4. Visible light (~700 nm red down to ~400 nm violet).
5. Ultraviolet (~400 nm down to ~10 nm).
6. X-rays (~10 nm down to ~0.01 nm).
7. Gamma rays — shortest wavelength (less than ~0.01 nm).

Mnemonic: Roman Men Invented Very Unusual X-ray Guns.

Important consequence of $v = f\lambda$. Because every EM wave has the same speed $c$ in a vacuum, multiplying the frequency by 10 must divide the wavelength by 10. The spectrum spans more than 18 orders of magnitude — that is why diagrams of it use a logarithmic scale.

Of the entire EM spectrum, the human eye can detect only a narrow band roughly 400 nm (violet) to 700 nm (red) — about one octave of frequency. We call this band visible light. To either side of it the radiation is invisible to us:

• Just below visible (longer wavelength, lower frequency) is infrared — we feel this as heat from a radiator but cannot see it.
• Just above visible (shorter wavelength, higher frequency) is ultraviolet — present in sunlight, responsible for tanning and sunburn.

The reason is biological, not physical. Retinal cells in our eyes contain pigments that change shape when struck by photons in this narrow energy range. Other animals have different pigments: bees can see ultraviolet; pit vipers can sense infrared.

Rainbow colours. Within the visible band, different wavelengths give different colours — the familiar spectrum of red, orange, yellow, green, blue, indigo, violet (longest to shortest wavelength). White light is a mixture of all of them. A prism or raindrop separates the colours by refracting each wavelength by a slightly different angle (covered in 4.6.2.2).

Why this matters in the exam. Examiners often ask you to explain why we cannot see X-rays even though they reach us. Mark schemes credit answers that link our limited vision to the narrow range of wavelengths the retina responds to, not to the X-rays' energy.

AQA wants you to know in outline how each kind of EM wave is generated. The further you go along the spectrum (from radio to gamma), the deeper inside the atom the source becomes:

Group	How it is typically produced
Radio	Oscillating electric currents in aerials and antennas
Microwave	Specialised oscillators (magnetrons, gunn diodes); also from cosmic background and warm objects
Infrared	Vibrating molecules in any object warmer than absolute zero
Visible	Electron transitions in heated or excited atoms
Ultraviolet	Higher-energy electron transitions; very hot objects (the Sun, a UV lamp)
X-rays	Rapid deceleration of fast electrons in heavy atoms (X-ray tube)
Gamma rays	Changes in the nucleus of an atom (radioactive decay)

The unifying rule is the AQA phrase: "electromagnetic waves are generated by changes in atoms and the nuclei of atoms". When an electron drops to a lower energy level it emits a photon of EM radiation. The energy gap determines the frequency:

$\text{photon energy} \propto f$

So a small jump (e.g. infrared in a molecule) gives low-frequency radiation; a giant jump (e.g. nuclear transition) gives gamma rays.

Cosmic origin. Many of these signals also arrive from space: radio from pulsars, microwaves from the cosmic background, infrared from dust clouds, visible from stars, UV/X-ray/gamma from hot stars and explosive events. The whole spectrum is in use by astronomers — telescopes are built for every band.

When an EM wave hits a boundary between two materials, the same three options apply as for any wave:

• Reflection. Some energy bounces back into the first medium. From a smooth surface (e.g. a mirror) reflection is specular and obeys $i = r$ (4.6.1.3).
• Absorption. Some energy is transferred to the particles of the second medium and warms it. A black surface absorbs almost all the visible light that hits it.
• Transmission. Some energy passes through into the second medium. If the wave hits the boundary at an angle other than 90°, the transmitted wave changes direction — this is refraction.

The proportions of reflection, absorption and transmission depend on:

• The materials on each side (a polished metal surface reflects far more than a sheet of paper).
• The wavelength of the wave (a thin film of oil can reflect blue light and transmit red).
• The angle the wave strikes the surface.

Why this matters in the lab. When you put a glass block on a piece of paper and shine a ray box at it, the bright ray you see emerging on the far side is the transmitted ray. A faint dim ray comes back off the front face (reflection). The slight warming of the block is absorption at work.

Refraction is the change of direction of a wave when it crosses a boundary between two materials in which it travels at different speeds.

Light slows down when it enters a denser optical medium (air → glass, air → water) and speeds up when it leaves (glass → air). If the ray hits the boundary at an angle, this change of speed forces the ray to bend:

• Entering a denser medium (slowing down): ray bends towards the normal — angle of refraction $r$ is smaller than angle of incidence $i$.
• Leaving a denser medium (speeding up): ray bends away from the normal — $r > i$.
• Hitting along the normal ($i = 0°$): no bending — the ray passes straight through, but its speed and wavelength still change inside the block.

Key conservation rule. The frequency of the wave does not change at the boundary — the source still vibrates at the same rate. From $v = f\lambda$, because $v$ falls and $f$ is fixed, $\lambda$ also falls inside the glass.

Examples in real life.

• A pencil dipped into a glass of water looks bent at the surface (light from the underwater part refracts as it leaves the water).
• A swimming pool looks shallower than it is, because rays from the bottom refract away from the normal as they leave the water.
• Lenses (in spectacles, cameras, microscopes) work by refracting different parts of the wave by different amounts.

To picture why a change in speed causes a change in direction, draw the light not as a ray but as a series of parallel wave-fronts (like ranks of soldiers marching shoulder to shoulder).

When the wave-front meets the boundary at an angle, one end of the front enters the new medium before the other. That end slows down (it's now in the denser medium) while the other end is still travelling at full speed in the original medium. The fast end therefore catches up — and the whole front rotates so that it travels in a new direction.

A helpful analogy is a car driving from a smooth road onto a muddy field at an angle. The wheel that hits the mud first slows down; the other wheel keeps going at the old speed; the car swerves into the field. Light does the same thing.

Because the front pivots towards the slower side, the ray (which is perpendicular to the front) bends towards the normal. The reverse happens on leaving the dense medium: the end that emerges first speeds up, the front rotates the other way, and the ray bends away from the normal.

Frequency must stay fixed. If you imagine counting wave-fronts crossing the boundary per second from each side, the number going in must equal the number coming out — otherwise wave-fronts would pile up at the boundary. So $f$ is conserved; only $v$ and $\lambda$ change.

Demonstration in the lab. In a ripple tank you can drop a glass plate in the water to make a shallower region. Plane water waves slow down over the plate and visibly bend towards the normal — exactly like light entering glass. This is the standard AQA Working Scientifically demo for refraction.

AQA's Required Practical 10 (Physics-only) is the standard refraction experiment for the GCSE Physics paper. The method is:

1. Place a rectangular glass block on a piece of paper and draw round it with a sharp pencil.
2. Use a ray box (a small lamp with a slit) to shine a thin ray of light onto one of the long sides of the block, at an angle of about 30°–60° from the normal.
3. Mark two dots along the incident ray (one near the ray box, one near the block) and two dots along the emerging ray on the far side.
4. Remove the block. Join the dots to draw the incident and emerging rays, then join the entry and exit points to reveal the path inside the glass.
5. Construct the normal (a line at 90° to the surface) at each refraction point with a set square.
6. Use a protractor to measure the angle of incidence $i$ and the angle of refraction $r$ at the first surface, with both angles taken from the normal.
7. Repeat for several different angles of incidence and plot $r$ against $i$.

What you should find. The graph is a smooth curve (not a straight line — the relationship is governed by Snell's law, which is post-GCSE). For every angle, $r < i$ at the air-to-glass boundary; the ray emerges from the second face parallel to the incident ray.

Common AQA mark-scheme phrases:

• "Use a sharp pencil" — fuzzy lines lose marks.
• "Measure angles from the normal" — measuring from the surface is the most common error.
• "Repeat for at least five angles" — for a smooth curve.
• "Identify the ray entering as the incident ray and the ray inside the glass as the refracted ray."

Every electromagnetic wave starts with an accelerating electric charge. The most common GCSE example is an aerial, but the same physics produces every part of the spectrum.

1. Radio waves from a transmitter aerial. An alternating current in the aerial pushes electrons up and down at the AC frequency. As the electrons accelerate they radiate an EM wave whose frequency exactly matches the AC frequency. A 100 MHz oscillation in an FM transmitter aerial radiates a 100 MHz radio wave. The wave's wavelength is $\lambda = v/f = (3\times10^8)/(1\times10^8) = 3\,\text{m}$ — handy for sizing aerials.

2. Radio waves absorbed by a receiver. When the radio wave arrives at the receiver aerial, its oscillating electric field forces the electrons in the aerial to oscillate at the same frequency. That induced AC is amplified by the receiver and demodulated into audio. So the rule is:

A radio wave can induce an alternating current with the same frequency as the wave in a conductor that absorbs it.

3. Visible light, UV, X-rays from atoms. When an outer electron in an atom drops from a higher energy level to a lower one, the surplus energy is emitted as a photon — typically a visible-light or UV photon. X-ray tubes use a faster process: high-speed electrons slam into a metal target and decelerate; the decelerating electrons radiate X-ray photons (bremsstrahlung).

4. Gamma rays from the nucleus. Gamma rays come from changes inside the atomic nucleus — usually after alpha or beta decay leaves the nucleus in an excited state (see 4.4.1.5). The nucleus rearranges and emits a high-energy gamma photon.

1. Oscillating charges in an aerial AC signal gen. e⁻ e⁻ radio waves

2. Changes within atoms / nuclei e⁻ photon electron drops to lower level → photon emitted

Two-way idea (mark-scheme phrase). EM waves can both create an oscillating current when they are absorbed by a conductor (the receiver) and be created by an oscillating current in a conductor (the transmitter). Both processes happen at the same frequency.

EM waves carry energy in discrete packets called photons. The energy of one photon depends only on the wave's frequency:

The higher the frequency, the more energy each photon carries.

The exact relation is $E = hf$ (Planck's relation), but you do not need this equation at GCSE — what you do need is the qualitative ordering.

Energy ordering of the EM spectrum (lowest energy on the left, highest on the right):

Radio → Microwave → Infrared → Visible → Ultraviolet → X-ray → Gamma

The wavelength runs the other way (radio waves are longest, gamma rays shortest), and the frequency increases as you go right. So radio waves are low frequency / long wavelength / low energy per photon; gamma rays are high frequency / short wavelength / high energy per photon.

Why this matters for safety. A single high-energy photon can knock an electron out of an atom — this is ionisation. Ionising photons damage molecules; in DNA this can cause mutation and cancer. The minimum photon energy to ionise atoms in human tissue is reached partway through the UV band, so UV-B, UV-C, X-rays and gamma rays are all ionising. Visible light, infrared, microwaves and radio are not.

That is why UV through gamma are biological hazards while radio waves (no matter how intense) are not ionising — even a very bright radio wave only warms tissue slightly, it cannot break a chemical bond on its own.

Ultraviolet (UV). Produced by the Sun and by 'black-light' lamps. UV damages skin cells; long-term over-exposure raises the risk of skin cancer and causes premature ageing of the skin. UV is also a leading cause of cataracts. UK protection advice: cover up, use SPF 30+ sunscreen, wear sunglasses with UV filters.

X-rays. Used in medical imaging (chest X-rays, dental X-rays, CT scans). X-rays pass easily through soft tissue but are absorbed by bone and dense materials, hence their use in imaging. Because they ionise, exposure carries a small mutation / cancer risk, so radiographers stand behind a lead screen and patients are exposed only briefly. Lead aprons protect the parts of the body not being imaged.

Gamma rays. Produced by nuclear decay. Used in radiotherapy to destroy cancer cells, and to sterilise surgical instruments. The same ionising property that kills cancer cells will damage healthy cells, so radiotherapy is carefully targeted with multiple beams crossing at the tumour.

Measuring exposure — the sievert (Sv). The unit of radiation dose is the sievert. 1 Sv represents a dose that gives a significant biological risk. Typical doses you should remember for AQA Higher Tier:

Source	Dose (mSv)
One chest X-ray	0.014 mSv
Dental X-ray	0.005 mSv
Flight London → New York	0.04 mSv
Annual UK background radiation	2.7 mSv
CT scan of the abdomen	~7 mSv
Fatal whole-body acute dose	≈ 5–10 Sv

1 Sv = 1000 mSv = 1,000,000 µSv. The sievert is a huge unit — most people never see anything close to 1 Sv in a lifetime.

ALARA principle. UK radiographers follow As Low As Reasonably Achievable: use the smallest dose that gives a useful image, shield surrounding tissue, and avoid unnecessary scans.

Use this table for any 'state the hazards of EM waves' question on Paper 2.

Wave	Source	Hazard	Why
Ultraviolet	Sun, UV lamps	Skin cancer, sunburn, cataracts	Ionises skin / eye cells
X-ray	X-ray tube	Mutation → cancer	Ionises living tissue, especially DNA
Gamma	Radioactive sources	Mutation → cancer; kills cells in high doses	Highest-energy ionising photons
Microwave (high-intensity)	Microwave oven, mobile mast (very high power)	Internal heating of tissue	Absorbed by water molecules
Infrared (very high intensity)	Lasers, hot objects	Burns skin / damages retina	Heats tissue

Note that low-power radio, microwave, IR and visible light at everyday intensities are not hazards — your phone's radio waves do not ionise.

AQA exam phrasing — when the question says 'explain the hazard', you should say what the radiation does (ionise / heat) and what the consequence is (cancer / burn / cataract). Two marks, two ideas.

Radio waves — TV and radio broadcasting.

• Long radio waves (>1 km) diffract strongly around hills and over the horizon. Used historically for long-wave AM radio (e.g. BBC Radio 4 LW at 198 kHz).
• Short-wave radio reflects off the ionosphere and can travel thousands of miles around the globe.
• VHF/UHF (TV, FM radio, DAB) travel in straight lines from line-of-sight transmitters such as Crystal Palace or Sutton Coldfield.

Radio waves work for broadcasting because they pass easily through air and diffract around buildings, and they can be generated by an alternating current at the broadcast frequency in a transmitter aerial (4.6.2.3).

Microwaves — satellite communications and microwave ovens.

• Satellite communications use microwaves with wavelengths a few centimetres long. They are not absorbed by the atmosphere or ionosphere, so they pass straight through to a geostationary satellite at 36,000 km altitude and back down again. This makes them perfect for TV, mobile-phone international calls and GPS.
• Microwave ovens use a different microwave frequency (≈ 2.45 GHz) tuned to be strongly absorbed by water molecules. Water molecules in food gain kinetic energy → the food heats from the inside as well as the outside.

The same wave (microwave) does two opposite jobs: in satellite work we want minimum absorption (so the wave reaches the satellite); in cooking we want maximum absorption (so the food heats).

Radio Microwave Infrared Visible UV X-ray Gamma

Broadcasting (TV, radio) Satellite comms / oven Remote control, cooking Vision, fibre-optic Fluorescent lamps, tanning Bone / lung imaging Radiotherapy, sterilising

long λ ← → short λ low f / low energy high f / high energy

Infrared (IR). IR is felt as heat. Anything warmer than absolute zero emits some IR.

• Cooking / electric heaters. Toasters and grills emit IR; the food absorbs it and warms.
• Thermal imaging cameras. Detect the IR emitted by warm bodies, used in night-vision goggles, by police searching for missing people, and to find heat leaks in poorly-insulated houses.
• TV remote controls. Emit short bursts of IR at the device's IR receiver — about 1–5 m range.
• Short-distance data links. IR LEDs are also used in fibre-optic comms (see below).

Visible light.

• Illumination and photography. The most obvious use — vision and photography depend on visible light.

• Optical-fibre communications. Modern broadband fibres use near-infrared and visible light to send data along glass fibres by total internal reflection (4.6.1.4). Visible / IR was chosen because:

  1. Glass fibres are extremely transparent to these wavelengths — very low absorption per kilometre.
  2. The wavelengths are short, so very high data rates (gigabits per second) are possible.
  3. The signal stays inside the fibre — no electrical interference from nearby cables.

The UK's domestic and trans-Atlantic data backbone runs on infrared-fibre optic cables under the sea.

Why not microwaves down a fibre? Microwaves have wavelengths of cm to m; they would not undergo total internal reflection in a glass strand only 10 µm wide. Visible / near-IR wavelengths (<1 µm) work brilliantly.

Energy-efficient fluorescent / LED lamps. Inside a fluorescent tube, an electric current passes through mercury vapour, exciting mercury atoms to emit UV photons. The UV strikes a phosphor coating on the inside of the glass; the phosphor absorbs the UV and re-emits visible light (a process called fluorescence). The tube is much more efficient than a filament bulb because it converts electrical energy directly to UV → visible, with very little wasted as heat.

Fluorescent security marking. Many UK banknotes and passports have a clear ink that emits visible light only when UV-illuminated. Shopkeepers check £20 notes with a small UV lamp — fake notes either have no fluorescent ink or the wrong pattern.

Sun-tanning lamps and UV phototherapy. UV-A is used in tanning beds; UV-B in medical phototherapy (e.g. for psoriasis). Both come with skin-cancer risk (see 4.6.2.3 HT).

Detecting biological contamination. Police forensic teams use UV lamps to make bodily fluids fluoresce at crime scenes.

The key property is the ability to cause fluorescence in certain materials — visible light is re-emitted when UV is absorbed. No other EM band does this in the same useful way.

X-rays in medical imaging.

X-rays pass easily through soft tissue (muscle, fat, lung) but are absorbed by dense materials such as bone and metal implants. By passing X-rays through the body and onto a photographic film or digital sensor, a shadow image of the bones (and any implants) is captured. This is why an X-ray is the first-line test for a suspected broken bone or a chest infection.

CT (computed tomography). A modern variant — the X-ray source rotates around the patient and a computer reconstructs a 3-D image slice by slice. Useful for soft-tissue imaging that plain X-rays can't show.

Industrial X-rays. Used to inspect welds, castings and aircraft fuselages for hidden cracks.

Gamma rays — sterilising and treating.

• Sterilising medical equipment. Surgical instruments and dressings are sealed in plastic packets and then irradiated with gamma rays. The high-energy photons kill any bacteria or viruses without raising the temperature — so the items can be sterilised inside their packaging.
• Sterilising food. Strawberries and pre-packed salads can be gamma-irradiated to extend shelf life. The food is not made radioactive by the gamma rays (gamma photons can't induce radioactivity in atoms at these energies).
• Radiotherapy. Multiple weak gamma beams cross at the tumour, where they add up to a dose strong enough to kill cancer cells. Healthy tissue around the tumour gets only one beam's worth, so it survives.
• Tracers in medicine. Technetium-99m is a gamma-emitting isotope injected to image organs (e.g. thyroid, kidney). Its 6-hour half-life is long enough for the scan but short enough to clear quickly.

Why gamma rays for these jobs? Three properties:

1. Very penetrating — pass through plastic packaging or human skin to reach the target.
2. Ionising — destroy DNA in bacteria and cancer cells.
3. Produced by simple radioactive sources (e.g. cobalt-60) — no big machine needed.

Examiner reports across 2022–2024 highlight a consistent issue: candidates state a use but never explain why. The mark scheme rewards a two-step answer.

Template.

Wave X is used for [use] because [physical property of wave X relevant to that use].

Examples.

Use	Property
Radio for broadcasting	Long wavelength → diffracts around obstacles
Microwaves for satellite comms	Passes through the atmosphere without much absorption
Microwaves in ovens	Strongly absorbed by water in food → heat
Infrared for remote controls	Easily directed in a beam over short distances
Infrared for thermal imaging	All warm objects emit IR
Visible / IR for optical fibres	Total internal reflection in thin glass at these wavelengths
UV for fluorescent lamps	Excites phosphor → visible light
X-rays for bone imaging	Absorbed by bone but pass through soft tissue
Gamma for sterilising	Penetrates packaging and kills bacteria by ionising
Gamma for radiotherapy	Ionising photons destroy cancer cells

For a 4-mark "explain two uses" question on Paper 2, write TWO of these "use because property" pairs. You'll get the full marks.