Progressive Waves

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe what is meant by wave motion as illustrated by vibration in ropes, springs and ripple tanks
  2. Understand and use the terms displacement, amplitude, phase difference, period, frequency, wavelength and speed
  3. Understand the use of the time-base and y-gain of a cathode-ray oscilloscope (CRO) to determine frequency and amplitude
  4. Derive, using the definitions of speed, frequency and wavelength, the wave equation v = fλ
  5. Recall and use v = fλ
  6. Understand that energy is transferred by a progressive wave
  7. Recall and use intensity = power/area and intensity ∝ (amplitude)² for a progressive wave

1. What is Wave Motion?

https://cambridgeassistant.com

Understanding Waves

A wave is a disturbance that transfers energy from one place to another without transferring matter (the actual material). Think of it like this: when you drop a stone in a pond, the water itself doesn't travel across the pond to the other side. Instead, the water particles move up and down in their own positions, but the wave pattern (and the energy) travels outward.

Key point: Particles on a wave vibrate about their mean (average) position. They are not carried along by the wave itself.

Wave Motion Illustrated by Vibrations

Wave motion can be demonstrated using different materials:

Vibrations in Ropes

When you move one end of a rope up and down, you create a wave that travels along the rope. The rope particles move perpendicular (at right angles) to the direction the wave is traveling. This creates what we call a transverse wave.

  • Direction of vibration: Up and down (perpendicular to wave travel)
  • Direction of energy transfer: Along the rope (horizontal)
  • What you see: Crests (high points) and troughs (low points) moving along the rope

Vibrations in Springs

When you push and pull one end of a spring back and forth, you create a wave where the spring coils move parallel (in the same direction) to the wave travel. This creates what we call a longitudinal wave.

  • Direction of vibration: Back and forth (parallel to wave travel)
  • Direction of energy transfer: Along the spring
  • What you see: Regions where coils are bunched together (compressions) and regions where they are spread apart (rarefactions)

Ripple Tanks

A ripple tank is a shallow tray of water with a light source above it. A paddle or dipper creates waves on the water surface. These waves can be seen as shadows on a screen below the tank.

  • The paddle produces waves on the water surface
  • Taller regions of water block more light, creating shadows
  • You can clearly see the wave pattern (wavefronts) moving across the water
  • Ripple tanks are useful for demonstrating wave properties like reflection, refraction, and diffraction

Important: Water waves in ripple tanks have characteristics of both transverse and longitudinal waves, but are primarily transverse.

Sign in to view full notes