Waves

Wave speed on a string depends on tension and linear mass density.

Sound is a compression-rarefaction wave; speed set by bulk modulus and density.

Sound travels fastest in solids, slower in liquids, slowest in gases.

Reflection law and Snell's law for refraction at a boundary.

Critical angle and the working principle of optical fibres.

Light as a transverse EM wave; Poynting vector as energy-flow density.

Two overlapping waves' displacements add — constructive or destructive interference.