4.6 Electromagnetic Waves

How EM waves are created

An accelerating charge creates a changing electric field, which produces a changing magnetic field, which in turn regenerates the electric field — a self-propagating electromagnetic (EM) wave. The E and B fields oscillate perpendicular to each other and to the direction of wave travel, making EM waves transverse waves that require no medium and travel at speed c in a vacuum.

The electromagnetic spectrum

All EM waves travel at c = 3.00 × 10⁸ m/s in a vacuum, related by c = fλ. They differ only in frequency and wavelength, ranging from low-frequency radio waves to high-frequency gamma rays (radio, microwave, infrared, visible, UV, X-ray, gamma). Higher frequency means higher photon energy (E = hf) and generally greater penetrating power.

Energy and momentum transport

EM waves carry energy and momentum even though they have no mass, which is why light can exert radiation pressure and why solar sails work. Intensity depends on the amplitude of the oscillating E and B fields; both fields carry energy in phase with each other as the wave propagates through space.

Key terms

How are EM waves generated?

By accelerating charges — the resulting changing E field creates a changing B field, and vice versa, propagating outward.

Speed of EM waves in vacuum

c = 3.00 × 10⁸ m/s, related to frequency and wavelength by c = fλ.

EM wave structure

Transverse wave with E and B fields oscillating perpendicular to each other and to the direction of propagation.

Photon energy equation

E = hf, where h is Planck's constant and f is frequency; higher frequency means higher energy.

Radiation pressure

Pressure exerted by EM waves because they carry momentum even though photons are massless.

Try an AP-style question

How are the electric and magnetic fields in an electromagnetic wave oriented?

1. Both along the direction of travel
2. Parallel to each other, perpendicular to travel
3. Parallel to each other and to the direction of travel
4. Perpendicular to each other and to the direction of travel