Lesson 1 of 10 · 7 min

Waves carry energy, not matter

NCERT §14.1

Behind Mamaji's instrument workshop is a temple tank. Riya tosses a pebble in, and rings spread across the still water. A cork floating a little way off rises and falls as each ring passes, and stays exactly where it was.

The story this chapter follows: Riya in the instrument workshop

Riya spends her holidays in her uncle's workshop, where he makes and mends sitars and bamboo pipes. A temple tank behind it, a bead string and a hand pump, the bench steel wire of 0.72 m and 5.0 g at 60 N, a 30.0 cm bamboo pipe and a sitar string tuned to 427 Hz carry the chapter, with NCERT's numbers throughout.

The lesson in notes

In short

Drop a pebble into a still pond and rings spread outward over the surface. A cork floating on the water only bobs up and down where it is; it does not ride out with the rings. The disturbance moves, the water stays.

A wave is a pattern of disturbance that travels without the medium as a whole travelling with it. Wind is air moving from place to place; sound is not.

Waves carry energy, and the pattern can carry information: speech, music, radio and light signals all travel as waves.

Mechanical waves, such as sound, water waves and seismic waves, obey Newton's laws and need a material medium: water, air, rock.

Electromagnetic waves, such as visible light, ultraviolet, radio, microwaves and X-rays, need no medium and cross a vacuum. In vacuum they all travel at the same speed c = 299,792,458 m s⁻¹.

Matter waves describe electrons, protons, neutrons and other particles; the electron microscope is an application of them.

The physics of waves grew out of the study of springs and pendulums in the seventeenth century; Christiaan Huygens (1629-1695), Robert Hooke and Isaac Newton are among the names linked with it.

A medium passes a disturbance on because its parts are coupled by elastic forces. Picture a chain of balls joined by springs: pull one end and each spring, as it stretches, tugs the next ball, so the disturbance spreads along the chain.

Air works the same way. Compressing a small region raises its density and pressure, that region pushes on the next, and a sound wave passes on as a travelling change of density. In a solid the atoms of the lattice behave like masses held by springs.