6.3 Electromagnetic Waves

What are electromagnetic waves?

Electromagnetic (EM) waves are transverse waves made of oscillating electric and magnetic fields, perpendicular to each other and to the direction of travel. Unlike sound, EM waves need no medium and travel through vacuum at the speed of light, c = 3.00 × 10⁸ m/s. All EM waves obey c = fλ.

The electromagnetic spectrum

The EM spectrum ranges from low-frequency, long-wavelength radio waves to high-frequency, short-wavelength gamma rays, passing through microwaves, infrared, visible light, ultraviolet, and X-rays. Higher frequency means higher photon energy (E = hf). Visible light is only a tiny slice, roughly 400–700 nm.

Production and properties

EM waves are produced by accelerating charges, such as electrons oscillating in an antenna. All EM waves carry energy and momentum, can be polarized, and exhibit reflection, refraction, diffraction, and interference — behaviors explored further in physical optics.

Key terms

Electromagnetic wave

A transverse wave of oscillating electric and magnetic fields, perpendicular to each other and to propagation direction; needs no medium.

Speed of light (c)

3.00 × 10⁸ m/s; the speed of all EM waves in vacuum.

c = fλ

Relates speed, frequency, and wavelength for any EM wave in vacuum.

EM spectrum order (low to high frequency)

Radio, microwave, infrared, visible light, ultraviolet, X-ray, gamma ray.

Cause of EM wave production

Accelerating (oscillating) electric charges, such as electrons in an antenna.

Try an AP-style question

What is the wavelength of a 100 MHz radio wave? (c = 3.0 × 10⁸ m/s)

1. 3.0 m
2. 3.0 × 10⁻⁸ m
3. 0.33 m
4. 30 m