How changing electric and magnetic fields travel as light
Maxwell showed that a changing electric field produces a magnetic field, just as a changing magnetic field produces an electric field. To make Ampère’s law consistent, he added the displacement current, ε₀ dΦE/dt, which flows even in the gap of a charging capacitor. As a result, oscillating charges send out electromagnetic waves in which electric and magnetic fields vibrate at right angles to each other and to the direction of travel, so the waves are transverse.
In a vacuum these waves travel at c = 1/√(μ₀ε₀) ≈ 3 × 10⁸ m/s and need no medium. The electromagnetic spectrum, in order of decreasing wavelength, is radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. All obey c = fλ, so higher frequency means shorter wavelength and more energetic photons. EM waves carry both energy and momentum.
Why it matters
Radio and mobile networks, microwave ovens, TV remotes, medical X-rays and cancer radiotherapy all use different parts of the spectrum. Knowing which waves are harmful, such as ultraviolet and gamma rays, explains the need for sunscreen and radiation shielding. This topic connects electricity, magnetism and optics into one picture of light.
Key terms
- Displacement current
- The term Maxwell added to Ampère’s law to account for a changing electric field producing a magnetic field.
- Transverse wave
- A wave in which the oscillations are perpendicular to the direction in which the wave travels.
- Electromagnetic spectrum
- The full range of electromagnetic waves arranged by wavelength or frequency.