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Spec mapping: OCR H556 Module 4.4 — Waves: the electromagnetic spectrum (radio through gamma); transverse nature; common speed c in vacuum; applications across regions. (Refer to the official OCR H556 specification document for exact wording.)
Of all the wave phenomena in physics, perhaps the most astonishing is that visible light — the radiation of familiar daylight — is only a very narrow slice of a much larger family of waves which all share the same fundamental properties. From the kilometre-long radio waves carrying radio broadcasts to the picometre-short gamma rays released from nuclear reactions, the electromagnetic spectrum is a single continuous family of transverse waves all travelling at the same speed in vacuum — the speed of light, c=3.00×108 m s−1.
This lesson summarises the regions of the spectrum, their properties and some of their uses, and develops the central physical fact that in vacuum all electromagnetic waves have the same speed regardless of frequency. That single statement — all EM waves at c in vacuum — is one of the great unifying facts of physics, and is the seed from which Einstein's special relativity grows (the constancy of c in every inertial frame).
In the nineteenth century, James Clerk Maxwell showed mathematically that oscillating electric and magnetic fields could self-propagate through empty space at a speed c given entirely by two electrical constants — the permittivity and permeability of free space:
c=ε0μ01≈3.00×108 m s−1
This result was one of the great triumphs of nineteenth-century physics. Maxwell realised that his theoretical prediction matched the measured speed of light (determined in the 1850s by Fizeau and Foucault) and concluded that light is an electromagnetic wave.
All electromagnetic waves consist of mutually perpendicular oscillating electric (E) and magnetic (B) fields, each perpendicular to the direction of propagation. They are therefore transverse waves. They need no medium: they can travel through empty space, which is why light from distant stars can reach us across billions of light-years of vacuum.
flowchart LR
subgraph Fields["Oscillating field picture"]
E[Electric field E, vertical]
B[Magnetic field B, horizontal]
P[Propagation direction]
end
E -- perpendicular --> B
E -- perpendicular --> P
B -- perpendicular --> P
P --> V[Speed in vacuum: c = 3 x 10^8 m/s]
All electromagnetic waves share the following properties. These are frequently tested.
Exam Tip: The value c=3.00×108 m s−1 is given in the OCR data, formulae and relationships booklet, but you should still know it by heart. Expect calculations to use this rounded value throughout the exam.
The full electromagnetic spectrum, in order of increasing frequency (and decreasing wavelength), is:
Radio → Microwave → Infrared → Visible → Ultraviolet → X-rays → Gamma rays
These boundaries are not sharp; they reflect the historical circumstances in which each region was discovered and are also influenced by the physical mechanism by which each type of wave is typically produced or detected. The table below summarises the typical wavelengths, frequencies and applications.
| Region | Wavelength (m) | Frequency (Hz) | Typical source | Typical use |
|---|---|---|---|---|
| Radio | >10−1 (m to km) | <3×109 | Oscillating electrons in aerials | Broadcasting, communications |
| Microwave | 10−3 to 10−1 | 3×109 to 3×1011 | Magnetrons, klystrons | Satellite links, radar, cooking |
| Infrared (IR) | 7×10−7 to 10−3 | 3×1011 to 4×1014 | Hot bodies | Thermal imaging, TV remote controls |
| Visible | 4×10−7 to 7×10−7 | 4×1014 to 7.5×1014 | The Sun, filament lamps, LEDs | Vision, photography |
| Ultraviolet (UV) | 10−8 to 4×10−7 | 7.5×1014 to 3×1016 | Very hot bodies, UV lamps | Fluorescence, sterilisation, sun-tanning |
| X-rays | 10−12 to 10−8 | 3×1016 to 3×1020 | Electron deceleration in X-ray tubes | Medical imaging, crystallography |
| Gamma rays | <10−12 | >3×1019 | Nuclear decay | Sterilisation, cancer therapy |
You are expected to know this order and to have a rough idea of wavelengths and frequencies in each region.
Exam Tip: A classic OCR question asks "list the regions of the electromagnetic spectrum in order of decreasing wavelength". The answer is: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Remember the mnemonic RMIVUXG — "Real Musicians Insist Very Useful X Gadgets" — or whatever you find memorable.
The visible spectrum — the only part of the electromagnetic spectrum our eyes can detect — runs from about 400 nm (violet) to about 700 nm (red). The rough colour order, from shortest to longest wavelength, is:
Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR or ROYGBIV in the reverse)
| Colour | Wavelength (nm) | Frequency (THz) |
|---|---|---|
| Violet | ~400 | ~750 |
| Blue | ~475 | ~630 |
| Green | ~510 | ~590 |
| Yellow | ~570 | ~525 |
| Orange | ~590 | ~510 |
| Red | ~700 | ~430 |
The human eye evolved to respond to exactly this range of wavelengths because it is the range in which the Sun emits most of its energy and in which Earth's atmosphere is most transparent. This is a nice example of cosmic coincidence and evolutionary fit.
Radio waves span the longest wavelengths in the spectrum — from a few millimetres up to many kilometres. They are produced when free electrons in a metal aerial oscillate back and forth, driven by an alternating current. Common applications include AM and FM broadcasting (long, medium and short wave), VHF and UHF television, mobile-phone signals (around 900 MHz to 2.6 GHz, depending on operator and band), Wi-Fi (at 2.4 GHz and 5 GHz), and astronomical radio observations of pulsars, hydrogen-line emission (1420 MHz) and the cosmic microwave background.
Earth's atmosphere is largely transparent to radio waves at frequencies above about 30 MHz; below that they are reflected by the ionosphere, an electrically conducting layer of charged particles at altitudes of 60 to 1000 km. This is why shortwave radio can travel long distances over the horizon: a wave radiated upwards bounces off the ionosphere and lands far away. Above the ionospheric cutoff (around 30 MHz), waves pass through into space, enabling satellite communication.
Microwaves occupy roughly 1 mm to 30 cm in wavelength. They are produced by magnetrons (the device inside every microwave oven) and by klystrons (used in satellite uplinks). Three important microwave applications appear in OCR contexts:
Infrared lies between 700 nm (just longward of red) and about 1 mm. Any object above absolute zero emits IR thermally (Stefan-Boltzmann: P=σAT4). At room temperature, the peak emission is around 10 μm (Wien's displacement law: λmaxT≈2.9×10−3 m K). Applications include thermal imaging (used in night-vision goggles, building-insulation surveys, medical diagnostics), TV remote controls (around 940 nm), heat lamps, and fibre-optic telecommunications (typically at 1310 nm or 1550 nm, where silica fibre is most transparent).
Visible light (400 to 700 nm) is the slice we see directly. Almost all of the imaging in human and animal vision, photography, microscopy and astronomy historically has been done in this band, and the entire field of geometric and physical optics has been developed around it.
UV (10 to 400 nm) is just shortward of visible. The Sun emits significant UV; most of it is absorbed by atmospheric ozone (UV-C and most UV-B) before reaching the ground, which is what makes the ozone layer biologically essential. UV-A (315 to 400 nm) reaches the ground and causes skin tanning (and, with extended exposure, skin damage and cancer). UV photons carry enough energy (E=hf of order 4 to 10 eV) to break some chemical bonds — they are ionising, in contrast to all longer-wavelength EM radiation.
X-rays (10−12 to 10−8 m) are produced in X-ray tubes by decelerating fast electrons (bremsstrahlung) or by characteristic atomic transitions in heavy metal targets. They penetrate soft tissue but are absorbed by bone and dense metals, which is what makes medical X-radiography work. X-ray crystallography (Bragg, 1913) uses the wave nature of X-rays to determine atomic positions in crystals — the technique that led to the structure of DNA (Franklin, Watson, Crick, 1953) and most of modern structural biology.
Gamma rays are the shortest-wavelength EM radiation, ≲10−12 m. They are produced by nuclear transitions (e.g. when a 60Co nucleus decays and the daughter nucleus de-excites). Gamma rays are highly penetrating and highly ionising; they are used in cancer radiotherapy (where a precisely targeted beam ablates a tumour) and in industrial sterilisation. The boundary between high-energy X-rays and gamma rays is a matter of origin rather than energy — X-rays come from electron processes, gamma rays from nuclear processes — but the photon energies overlap above about 100 keV.
Earth's atmosphere is not equally transparent at all wavelengths. There are two main atmospheric windows of high transparency:
In between these windows, the atmosphere is mostly opaque: ultraviolet is absorbed by the ozone layer (essential for life — high-energy UV is mutagenic); much of the infrared is absorbed by water vapour, carbon dioxide and methane (this absorption is the greenhouse effect, and increases in these gases enhance the absorption and warm the lower atmosphere); X-rays and gamma rays are absorbed in the upper atmosphere by interaction with atmospheric atoms (which is fortunate, given how ionising they are).
This atmospheric opacity is why space telescopes are scientifically important. The Hubble Space Telescope (visible/UV/near-IR) and the James Webb Space Telescope (infrared) avoid atmospheric absorption entirely. Chandra (X-ray) and Fermi (gamma ray) observe wavelengths that are simply impossible from the Earth's surface.
The narrow band of visible light at which life evolved to see is no accident: it is the optical window for the Sun's surface temperature (∼5800 K) at which the atmosphere is most transparent. Wien's displacement law, λmaxT=2.9×10−3 m K, places the Sun's spectral peak at about 500 nm — squarely in the green. Our eyes evolved at exactly this wavelength because it is the brightest natural illumination available on Earth's surface.
Q. A radio station broadcasts at 98.8 MHz. Calculate the wavelength in metres.
A. Using v=fλ with v=c=3.00×108 m s−1:
λ=fc=98.8×1063.00×108=3.04 m
Q. A green laser pointer emits light of wavelength 532 nm. Calculate the frequency.
A. 532 nm =5.32×10−7 m, so:
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