Waves, sound and light

A wave transfers energy without transferring matter. In a transverse wave the oscillation is perpendicular to the travel direction, as in light and water surface waves. In a longitudinal wave it is parallel, as in sound, which travels as compressions and rarefactions.

Four quantities describe any wave: wavelength, frequency, amplitude and speed. Speed equals frequency times wavelength, so in a given medium raising frequency shortens wavelength. Amplitude is separate and sets intensity, which is loudness for sound and brightness for light.

• Reflection: the wave bounces, with the angle of incidence equal to the angle of reflection.
• Refraction: the wave changes speed entering a new medium and therefore bends; light slows and bends toward the normal entering glass.
• Total internal reflection: beyond a critical angle nothing escapes, which is the principle behind optical fibres.
• Diffraction: waves spread through gaps and around edges, most noticeably when the gap is comparable to the wavelength.
• Interference: overlapping waves add, reinforcing where crests meet and cancelling where a crest meets a trough.

Radio, microwave, infrared, visible, ultraviolet, X-ray and gamma are one continuous family, all travelling at the same speed in a vacuum and differing only in frequency and wavelength.

Higher frequency means higher energy per photon, which is why the hazards change along the spectrum. Radio and microwave mostly heat; ultraviolet and above carry enough energy per photon to break chemical bonds and damage DNA, which is what makes them ionising.

Sound needs a medium, so it cannot cross a vacuum. It travels faster in solids than liquids and faster in liquids than gases, because tightly coupled particles pass the disturbance along more quickly.

Pitch corresponds to frequency and loudness to amplitude. The Doppler effect shifts the observed frequency when source and observer move relative to each other, raising pitch on approach and lowering it on retreat. The same effect applied to light is how astronomers established that distant galaxies are receding.

Light diffracts and interferes, which only waves do. Yet the photoelectric effect shows that whether electrons are emitted from a metal depends on frequency, not intensity: dim blue light works where bright red light does not. That requires light to arrive in discrete quanta with energy set by frequency.

Both descriptions are needed and neither alone is sufficient. Matter shows the same duality: electrons produce interference patterns in double-slit experiments. This is not a gap waiting to be resolved, it is the actual behaviour of the world at small scales.