Physics (9630) | Longitudinal and transverse waves

Welcome to Waves: Longitudinal and Transverse

Hello future physicists! This chapter is all about understanding how waves move energy from one place to another. Don't worry if the vocabulary seems dense—we are simply going to look at the two main ways that the medium (the stuff the wave travels through) moves, and how that relates to the wave's direction. Mastering this distinction is key to understanding everything from sound to light!

Key Takeaway: All waves transfer energy, not matter. The way the medium oscillates defines the wave type.

3.5.4 The Two Natures of Waves

Before diving into the two types, let's remember the basic idea of a wave: it's a disturbance that travels, carrying energy.

The key distinction between our two wave types is the relationship between:
1. The direction of oscillation (how the particles or fields move).
2. The direction of energy propagation (the direction the wave, and its energy, travels).

1. Transverse Waves

Transverse waves are the ones that look like waves in cartoons—a classic sine shape.

What defines a Transverse Wave?

In a transverse wave, the oscillation or displacement of the particles (or fields) of the medium is perpendicular (at a 90° angle) to the direction of energy propagation.

• Displacement: Up and down (or side to side).
• Propagation: Forward.
• Key Points: The wave consists of crests (peaks) and troughs (valleys).

Analogy: The stadium "Wave"

Imagine you are doing the "wave" at a sports stadium. You move up and down (oscillation), but the wave itself travels horizontally around the stadium (propagation). The directions are perpendicular!

Examples of Transverse Waves

The most important examples to remember are:

1. Waves on a String: If you flick a taut rope, the segments of the rope move up and down, but the pulse travels along the rope.
2. Electromagnetic Waves (EM): All electromagnetic radiation—radio waves, microwaves, visible light, X-rays, gamma rays—are transverse waves. This is because the oscillating electric and magnetic fields are perpendicular to the direction of travel.

Key Takeaway: If the particle movement is 90° to the wave movement, it's transverse.

2. Longitudinal Waves

Longitudinal waves are slightly less intuitive because the medium doesn't move "up and down."

What defines a Longitudinal Wave?

In a longitudinal wave, the oscillation or displacement of the particles of the medium is parallel (in the same direction) to the direction of energy propagation.

• Displacement: Back and forth (parallel to propagation).
• Propagation: Forward.
• Key Points: The wave consists of regions of high pressure/density called compressions, and regions of low pressure/density called rarefactions.

Analogy: The Slinky Push

If you push and pull one end of a Slinky (coiled spring) along the ground, the coils move back and forth (oscillation). The energy pulse travels forward along the Slinky (propagation). The movement is parallel!

Example of Longitudinal Waves

The key example here is:

Sound Waves: Sound waves travel by causing air molecules to bunch up (compression) and spread out (rarefaction). The molecules are moving back and forth in the same direction the sound is travelling.

Key Takeaway: If the particle movement is parallel to the wave movement, it's longitudinal.

3. Important Properties and Connections

A. Speed of Electromagnetic Waves

A critical fact derived from Maxwell's equations is that all electromagnetic waves (light, radio, etc., which are transverse waves) travel at the same speed when in a vacuum.

This speed is the speed of light, \(c\):
\(c = 3.00 \times 10^8 \text{ m s}^{-1}\) (in a vacuum).

In other media, the speed will decrease, but in the vacuum of space, they are all constant.

B. Application: Ultrasound in Medicine

The syllabus requires knowledge of the use of ultrasound in medicine.

What is ultrasound? It is simply sound (a mechanical wave) with a frequency higher than the human ear can detect (above 20 kHz). Since sound is a mechanical wave, ultrasound waves are longitudinal.

Use in Medicine: Ultrasound transducers send pulses into the body. These pulses reflect off boundaries between different tissues (like muscle and bone). By timing these echoes, a detailed image of internal structures can be built up. (It is generally safer than X-rays as it uses non-ionising radiation.)

4. Polarisation: Proof of Transverse Waves

This is one of the most important concepts in the waves topic, as it definitively proves that light (and all EM waves) must be transverse.

What is Polarisation?

Polarisation refers to restricting the oscillations of a transverse wave to only one plane.

Step 1: Unpolarised Waves

Normally, light waves (or waves on a string) are unpolarised. This means the oscillations are happening in all possible planes that are perpendicular to the direction of propagation.
Example: A light bulb emits photons vibrating horizontally, vertically, and diagonally simultaneously.

Step 2: Polarising the Wave

A polariser (like a filter) acts like a tiny grid or sieve. It only allows oscillations that are parallel to its transmission axis to pass through.

The resulting wave is plane-polarised (or linearly polarised), meaning the oscillation is restricted to a single plane.

Analogy: The Fence and the Rope

Imagine you are sending waves down a rope.
1. If you shake the rope randomly (vertically, horizontally, etc.), that's unpolarised.
2. If you pass the rope through a narrow, vertical picket fence (the polariser), only the vertical oscillations can get through. The wave emerging is plane-polarised in the vertical plane.

Crucial Point: Longitudinal waves CANNOT be polarised. Since longitudinal waves oscillate parallel to the direction of propagation, there is no separate plane of oscillation to restrict. If you try to polarise sound, nothing happens!

Applications of Polarisers

1. Polaroid Material (Sunglasses)

Light reflecting off horizontal surfaces (like water or tarmac) tends to become partially polarised in the horizontal plane. This horizontal light is responsible for intense glare.

Polaroid sunglasses are made with vertical transmission axes. They block the horizontally polarised glare while allowing the non-polarised useful light to pass through, improving visibility.

2. Alignment of Aerials (Transmission and Reception)

For radio and television signals (which are EM waves, hence transverse), the transmitting aerial sends out waves that are usually polarised (e.g., vertically or horizontally).

For the receiving aerial to pick up the maximum signal, it must be aligned parallel to the plane of polarisation of the transmitted wave. If the aerials are perpendicular, the signal strength will be minimal.

Final Review: Longitudinal vs. Transverse

You've successfully covered the core characteristics of wave motion! The ability to distinguish these two types and understand how polarisation proves the nature of light are vital skills for your exams. Keep practising those definitions!