electromagnetic radiation

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Electromagnetic radiation is propagating oscillations of electric and magnetic fields. They carry energy and momentum without the need for a medium. The spectrum extends from long radio waves to gamma rays. Visible light is just a narrow band in this range.

History

James Clerk Maxwell theoretically predicted such waves in the 19th century, and Heinrich Hertz later created them in the laboratory. Scientists realized that visible light is also an electromagnetic wave.

How it works

If you shake an electric charge, it creates a changing electric field around it, which in turn generates a magnetic one, and so on in a chain — the wave runs forward. It’s similar to jerking a rope: the impulse travels along it.

💡 A microwave oven uses radiation with a wavelength of about 12 cm, causing water molecules to rotate and heat food, but a dry plate hardly heats up.

Electromagnetic radiation is a disturbance of the electromagnetic field, propagating in vacuum at the speed of light c ≈ 3×10⁸ m/s. Classically described by Maxwell's equations, from which a wave equation for fields E and B is derived. Solutions are transverse waves: vectors E and B are perpendicular to each other and to the direction of propagation. Quantum mechanically, radiation consists of photons with energy E = hν and momentum p = h/λ. Spectrum: radio (10³–10⁻¹ m), microwaves (10⁻¹–10⁻³ m), infrared (10⁻³–7×10⁻⁷ m), visible (7×10⁻⁷–4×10⁻⁷ m), ultraviolet (4×10⁻⁷–10⁻⁸ m), X-ray (10⁻⁸–10⁻¹² m), gamma (<10⁻¹² m).

History

James Clerk Maxwell in 1865 unified electricity and magnetism, predicting electromagnetic waves whose speed matched the speed of light, indicating the electromagnetic nature of light. Heinrich Hertz in 1887 first experimentally produced radio waves. Wilhelm Röntgen discovered X-rays in 1895. Max Planck (1900) and Albert Einstein (1905) laid the quantum theory, explaining the photoelectric effect. Frederick Herschel discovered infrared radiation as early as 1800.

How it works

According to Maxwell's equations, a changing magnetic field generates a vortex electric field, and vice versa, creating a self-sustaining wave. The energy of the wave is proportional to the square of the amplitude and frequency (Planck's formula for photons). The Doppler effect gives a red or blue shift when the source moves. Interaction with matter: absorption, reflection, refraction. Spectroscopy uses the decomposition of radiation to analyze the composition of substances.

\nabla^2 \mathbf{E} = \frac{1}{c^2} \frac{\partial^2 \mathbf{E}}{\partial t^2}

E — electric field intensity vector, c — speed of light in vacuum, c ≈ 3×10⁸ m/s, ∇² — Laplace operator (sum of second spatial derivatives)

E = h\nu = \frac{hc}{\lambda}

E — photon energy, h — Planck constant, h ≈ 6.626×10⁻³⁴ J·s, ν — radiation frequency, c — speed of light, λ — wavelength

c = \frac{1}{\sqrt{\mu_0 \varepsilon_0}}

c — speed of light in vacuum, μ₀ — magnetic constant, μ₀ ≈ 4π×10⁻⁷ H/m, ε₀ — electric constant, ε₀ ≈ 8.854×10⁻¹² F/m

Open problems & fun facts

Hierarchy problem: why is the electromagnetic force many orders of magnitude stronger than gravity?; Nature of quantum entanglement in electromagnetic interactions; Interaction of light with metamaterials (e.g., for invisibility); Quantum decoherence of electromagnetic states in hot environments

💡 The electromagnetic field has zero rest mass, so photons always fly at the speed of light, and time does not flow for them.

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electromagnetic radiation

History

How it works

History

How it works

History

How it works

Open problems & fun facts

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