List I (Wave)
List II (Source)
Microwave → IV. Klystron / Magnetron valve
Microwaves are generated by specially designed vacuum tubes. The klystron bunches an electron beam using electric fields and extracts microwave energy from the bunched beam. The magnetron uses a magnetic field to spiral electrons past resonant cavities. Magnetrons power microwave ovens at 2.45 GHz; klystrons are used in radar and satellite communication.
X-ray → I. Inner shell electron transitions in heavy atoms
When high-energy electrons bombard heavy metal atoms (tungsten, molybdenum), they knock out electrons from inner shells (K, L). Outer shell electrons fall in to fill the vacancy and release the energy difference as X-ray photons. These are called characteristic X-rays and have discrete wavelengths specific to each element.
Gamma ray → II. Radioactive decay of nucleus
Gamma rays originate within the atomic nucleus. After alpha or beta decay, the daughter nucleus is left in an excited energy state. It transitions to the ground state by emitting a gamma-ray photon — the highest energy, highest frequency, shortest wavelength radiation in the EM spectrum.
Infrared → III. Hot bodies / vibrating molecules
All objects above absolute zero emit infrared radiation — the higher the temperature, the more intense the emission and the shorter the peak wavelength (Wien's law). Vibrating polar molecules in gases also absorb and emit IR. Our bodies, the sun, hot food, and electric heaters all emit infrared radiation.
James Clerk Maxwell theoretically predicted electromagnetic waves in 1865 by adding the concept of displacement current to Ampere's law. He showed that a changing electric field produces a magnetic field, and a changing magnetic field produces an electric field — together they sustain each other and propagate as a wave through vacuum at speed c = 1/√(μ₀ε₀) = 3×10⁸ m/s. Heinrich Hertz experimentally confirmed EM waves in 1888 using oscillating circuits.
In an EM wave, the electric field E and magnetic field B are always perpendicular to each other and to the direction of propagation. They oscillate in phase — reaching maximum and minimum values at the same instant. The amplitude relationship is E₀ = cB₀. EM waves are transverse waves and can be polarised.
| Wave | Wavelength | Source | Uses |
|---|---|---|---|
| Radio | >0.1 m | Oscillating circuits | AM/FM, TV broadcast |
| Microwave | 1mm–0.1m | Klystron, magnetron | Radar, oven, WiFi, satellite |
| Infrared | 700nm–1mm | Hot bodies, molecules | Remote control, night vision |
| Visible | 400–700nm | Outer shell transitions | Vision, photography |
| UV | 10–400nm | Very hot bodies, arcs | Sterilisation, vitamin D |
| X-ray | 0.01–10nm | Inner shell transitions | Medical imaging, security |
| Gamma | <0.01nm | Nuclear decay | Cancer therapy, PET scan |
📌 Speed in vacuum: c = 3×10⁸ m/s (same for ALL EM waves)
📌 Relation: c = νλ, E = hν = hc/λ (photon energy)
📌 Transverse waves — can be polarised
📌 E ⊥ B ⊥ direction of propagation
📌 E₀ = cB₀ (amplitude relationship)
📌 Travel through vacuum — no medium needed
📌 Carry energy and momentum
📌 Not deflected by electric or magnetic fields (no charge)
📌 Show reflection, refraction, diffraction, interference, polarisation
Maxwell noticed a contradiction in Ampere's law — between capacitor plates, no conduction current flows but the law incorrectly predicted no magnetic field there. He added the displacement current to fix this:
I_d = ε₀ × dΦ_E/dt
Modified Ampere's Law: ∮ B·dl = μ₀(I + I_d)
This was revolutionary — it showed that a changing electric field produces a magnetic field even in vacuum. Combined with Faraday's law (changing B produces E), this creates a self-sustaining oscillation: the propagating EM wave. Maxwell calculated the speed of this wave as 1/√(μ₀ε₀) = 3×10⁸ m/s — exactly the measured speed of light — proving light is an EM wave.
EM waves carry energy. The energy density (energy per unit volume) in an EM wave is equally divided between electric and magnetic fields: u = ε₀E² = B²/μ₀. Intensity (power per unit area) I = uc = ε₀cE₀²/2 = E₀B₀/(2μ₀). The radiation pressure exerted by EM waves: P = I/c (complete absorption) or P = 2I/c (complete reflection). This radiation pressure pushes comet tails away from the sun and can propel spacecraft (solar sails).
Every object emits EM radiation — the spectrum depends on temperature. Wien's displacement law: λ_max × T = 2.898×10⁻³ m·K. Hotter objects emit peak radiation at shorter wavelengths. Sun (5778 K) → peak at ~500 nm (visible green). Human body (310 K) → peak at ~9.3 μm (infrared). Electric bulb filament (2700 K) → peak at ~1100 nm (near IR) with some visible.
λ_max × T = 2.898×10⁻³ m·K (Wien's Law)
Total power ∝ T⁴ (Stefan-Boltzmann Law: P = σAT⁴)
Microwave applications: Radar (detect aircraft, speed guns), microwave communication (satellite, mobile phone towers), WiFi and Bluetooth (2.4 GHz band), and cooking (2.45 GHz resonance with water molecules).
X-ray applications: Medical radiography (bones absorb X-rays more than soft tissue), CT scans (computed tomography), crystallography (X-ray diffraction reveals molecular structure — DNA double helix was discovered this way), and airport security scanners.
Gamma ray applications: Cancer radiotherapy (focused gamma rays destroy tumour cells), PET scans (positron emission tomography), sterilisation of medical equipment, and food irradiation to kill bacteria without using heat.
Different chemical bonds vibrate at characteristic infrared frequencies — like fingerprints. IR spectroscopy identifies molecules by their absorption spectrum. Greenhouse gases (CO₂, CH₄, H₂O) absorb IR emitted by Earth's warm surface and re-emit it back, warming the planet. Without any greenhouse effect, Earth would be ~33°C colder. Increased CO₂ from burning fossil fuels enhances this effect, causing global warming.