Waves & The Electromagnetic SpectrumEdexcel IGCSE Physics: Revision notes

A wave transfers energy and information from one place to another without transferring matter — the particles or field of the medium oscillate about a fixed position but do not travel with the wave.

• Transverse waves: vibrations are perpendicular (at 90°) to the direction of energy transfer, e.g. light, all electromagnetic waves, water waves
• Longitudinal waves: vibrations are parallel to the direction of energy transfer, forming compressions and rarefactions, e.g. sound

Key wave definitions:

• Amplitude: the maximum displacement of a point on the wave from its undisturbed (rest) position
• Wavefront: a line or surface joining points on a wave that are in phase (e.g. all at a crest)
• Frequency: the number of complete waves passing a point per second, measured in hertz (Hz)
• Wavelength: the distance from one point on a wave to the identical point on the next wave (e.g. crest to crest)
• Period: the time taken for one complete wave cycle

All waves obey the wave equation, relating speed, frequency and wavelength:

$v = f \times \lambda$

where $v$ is speed (m/s), $f$ is frequency (Hz) and $\lambda$ is wavelength (m).

Frequency and time period are also linked:

$f = \dfrac{1}{T}$

All waves — including all electromagnetic waves — can be reflected and refracted.

When a wave source moves relative to an observer, the observer detects a change in the wave's observed frequency and wavelength compared to when the source is stationary. This is the Doppler effect.

• If the source moves towards the observer, the observed wavelength decreases and the observed frequency increases
• If the source moves away from the observer, the observed wavelength increases and the observed frequency decreases

The Doppler effect applies to all types of wave, including sound (e.g. the pitch of a passing ambulance siren) and light (e.g. red-shift from receding galaxies).

Light is part of a continuous electromagnetic (EM) spectrum. All EM waves travel at the same speed in free space (a vacuum). The full spectrum, in order of decreasing wavelength / increasing frequency, is:

Within visible light, the colours run from red (longest wavelength) to violet (shortest wavelength).

Uses of EM radiation:

Dangers of excessive exposure:

• Microwaves: internal heating of body tissue
• Infrared: skin burns
• Ultraviolet: damage to surface cells, blindness
• Gamma rays: cancer, mutation

Simple protective measures include limiting exposure time, using shielding (e.g. lead for gamma/X-rays), and wearing protective clothing or sunscreen (for UV).