Properties of electromagnetic waves

Common properties of all EM waves

All seven members of the EM spectrum (radio, microwave, infrared, visible, ultraviolet, X-ray, gamma) share these properties:

• Transverse waves — oscillations are perpendicular to the direction of energy travel.
• Travel at the same speed in a vacuum: 3 × 10⁸ m/s.
• Can travel through a vacuum (unlike sound). This is why light from the Sun reaches Earth across empty space.
• Transfer energy from a source to an absorber.
• Obey the wave equation v = f λ.

Speed in different media

Although the speed in vacuum is fixed, EM waves slow down slightly in transparent media (glass, water). This change in speed at a boundary causes refraction.

Frequency, wavelength and energy

• Across the EM spectrum: as frequency increases, wavelength decreases.
• Higher-frequency waves carry more energy per photon.
• Order of increasing frequency / energy: radio < microwave < infrared < visible < ultraviolet < X-ray < gamma.

Emission and absorption

EM waves are emitted and absorbed by atoms when their electrons change energy levels:

• Emission: an electron drops from a higher to a lower energy level, releasing a photon of EM radiation.
• Absorption: an electron jumps from a lower to a higher energy level by absorbing a photon.

The frequency / energy of the photon depends on the size of the energy gap. This is why hot objects emit characteristic spectra.

Wave equation worked example

A radio station broadcasts at 100 MHz (1.0 × 10⁸ Hz). Calculate the wavelength.

λ = v ÷ f = (3 × 10⁸) ÷ (1.0 × 10⁸) = 3 m.

Edexcel exam tip

A common 1-mark item is "give one property of all EM waves". Safe answers: "they are all transverse"; "they all travel at 3 × 10⁸ m/s in a vacuum"; "they can all travel through a vacuum"; "they all transfer energy". Don't write "they have different frequencies" — that's a difference, not a shared property. Also, the speed of light is 3 × 10⁸ m/s, not 3 × 10⁶ — losing the exponent loses the mark.