Waves — AQA Combined Science: Trilogy

Waves transfer energy and information without transferring matter. This unit covers wave properties, the wave equation and the electromagnetic spectrum.

Transverse and longitudinal waves

Transverse waves — the oscillations are perpendicular (at 90°) to the direction of energy transfer. Examples: all electromagnetic waves, ripples on water, waves on a rope.
Longitudinal waves — the oscillations are parallel to the direction of energy transfer, creating compressions and rarefactions. Example: sound waves.

In both cases the wave transfers energy, but the particles (or fields) only oscillate about a fixed point — matter is not transferred.

Describing waves

Amplitude — maximum displacement from the rest position.
Wavelength (λ) — distance between two corresponding points on adjacent waves (e.g. crest to crest), in metres.
Frequency (f) — number of waves passing a point per second, in hertz (Hz).
Period (T) — time for one complete wave: $$T = \frac{1}{f}$$

The wave equation

$$v = f \times \lambda$$ (wave speed = frequency × wavelength)

Wave speed is measured in m/s. For all waves, if the speed is constant, increasing the frequency means decreasing the wavelength.

Required practical: measuring the frequency, wavelength and speed of waves in a ripple tank (water waves) and on a stretched string.

Wave behaviour at boundaries

When a wave meets a boundary between two materials it can be:

reflected (bounces back),
transmitted (passes through, often refracted — changing direction because its speed changes), or
absorbed (energy transferred to the material).

In reflection, the angle of incidence equals the angle of reflection (measured from the normal).

The electromagnetic spectrum

Electromagnetic (EM) waves are transverse waves that all travel at the same speed through a vacuum (the speed of light). They form a continuous spectrum, arranged by wavelength and frequency:

Radio → Microwave → Infrared → Visible light → Ultraviolet → X-rays → Gamma rays

(longest wavelength/lowest frequency → shortest wavelength/highest frequency)

Our eyes can only detect the narrow visible light part.

Uses

Radio waves — TV and radio broadcasting, communications.
Microwaves — cooking, satellite and mobile phone communications.
Infrared — cooking, heaters, thermal imaging, remote controls, optical fibres.
Visible light — seeing, photography, illumination.
Ultraviolet — energy-efficient lamps, sun tanning, detecting forgeries.
X-rays — medical imaging of bones, security scanners.
Gamma rays — sterilising equipment and food, treating cancer (radiotherapy).

Hazards

The higher-frequency waves are ionising and can damage cells:

Ultraviolet can cause skin to age prematurely and increase the risk of skin cancer.
X-rays and gamma rays are ionising and can cause mutation of genes and cancer.

Radiation dose (in sieverts) measures the risk of harm from radiation.

How EM waves are produced

Radio waves can be produced by oscillations in electrical circuits. When EM waves are absorbed they may create an alternating current with the same frequency, which is how radio signals are received. Changes in atoms and their nuclei can generate and absorb EM radiation over a wide frequency range (e.g. gamma rays from the nucleus).

Exam tips

Distinguish transverse and longitudinal waves with examples (light vs sound).
Learn v = fλ and T = 1/f and practise rearranging and converting units (kHz, MHz).
Memorise the EM spectrum in order and the uses and hazards of each region.
Remember all EM waves travel at the same speed in a vacuum and are transverse.
The ionising (dangerous) waves are UV, X-rays and gamma.

Physics: Waves