Revise Transverse and longitudinal waves for Cambridge 9702 (7.2)

At its core, a wave is a disturbance that transfers energy through a medium or space. Critically, while the energy travels, the particles of the medium itself only oscillate about their equilibrium positions; they do not get carried along with the wave. This distinction is vital for understanding how waves work.

In a transverse wave, the oscillations of the particles in the medium are perpendicular to the direction in which the wave's energy is travelling. Imagine shaking one end of a rope up and down; the wave moves horizontally along the rope, but each part of the rope only moves vertically.

Conversely, in a longitudinal wave, the particles of the medium oscillate parallel to the direction of energy transfer. Picture pushing and pulling on a Slinky spring; the disturbance moves along the spring, and each coil moves back and forth in the same direction.

To describe waves quantitatively, we use several fundamental properties. Wavelength (λ) is the distance between two identical points on consecutive waves. Frequency (f) is how many full waves pass a point per second, while Period (T) is the time for one complete wave to pass.

The relationship between period and frequency is: $T = \frac{1}{f}$.

The universal wave equation links speed, frequency, and wavelength: $v = f\lambda$.

Polarisation is a special characteristic where the oscillations of a transverse wave are restricted to a single plane. Imagine light waves oscillating in all directions perpendicular to their travel path. A polarising filter acts like a vertical fence, only allowing oscillations in the vertical plane to pass through.

For polarised light, Malus' Law describes the intensity after passing through a second filter, where $I_0$ is the initial intensity and $\theta$ is the angle between the wave's plane of oscillation and the polariser's axis. $I = I_0\cos^2\theta$