Electromagnetic waves

Electromagnetic (EM) waves are transverse waves of oscillating electric and magnetic fields. They:

• Travel through a vacuum (no medium needed).
• All travel at the same speed in a vacuum: $c = 3 \times 10^8$ m/s.
• Vary across a continuous spectrum from radio to gamma.

The seven categories

In order of increasing frequency (and decreasing wavelength):

1. Radio waves — longest λ (km to mm). Communication: TV, radio.
2. Microwaves — mm to cm. Cooking, satellite communication, radar.
3. Infrared (IR) — μm. Thermal imaging, remote controls, optical fibres.
4. Visible light — 400 nm (violet) to 700 nm (red). The narrow window we can see.
5. Ultraviolet (UV) — sun-tan, fluorescence, sterilisation.
6. X-rays — medical imaging, security scanning.
7. Gamma rays — nuclear processes, cancer treatment.

A useful mnemonic: Radio, Micro, Infra, Visible, Ultra, X, Gamma.

Properties

• All EM waves are transverse — oscillating perpendicular to direction of travel.
• All travel at $c = 3 \times 10^8$ m/s in a vacuum.
• All carry energy — higher frequency = more energy per photon.
• All can be reflected, refracted, diffracted and absorbed at boundaries.

Sources

• Radio waves: oscillating currents in antennas; produced when electrons accelerate in conductors.
• Microwaves: maser cavities, magnetrons.
• IR: thermal radiation from any warm object.
• Visible: hot bodies, electron transitions.
• UV: very hot bodies (sun), fluorescent tubes.
• X-rays: rapidly decelerating electrons.
• Gamma: nuclear decay.

Hazards

Higher-energy EM (UV, X-rays, gamma) is ionising — can damage cells and DNA, causing cancer or burns. Lower-energy EM (radio, microwave) is non-ionising and lower risk, though microwaves can cause internal heating.