Lesson 3: Waves behave in predictable ways

Lesson Focus

Waves don't just travel in a straight line. This lesson explores how waves predictably change direction, speed, and form when they hit barriers, enter new materials, or meet other waves.

Learning Objectives

• Describe how waves change direction when they encounter a barrier.
• Explain what happens to a wave when it enters a new medium.
• Identify the ways that waves can interact with one another.
• Investigate how waves behave when they meet a barrier in an experiment.

When a wave hits a barrier it cannot pass through, it bounces back. This is reflection. The barrier exerts an opposite force, sending the wave's energy in a new direction. An echo is the reflection of sound waves, while a mirror uses reflected light waves. What makes an echo different from the original sound?

A wave bends when it enters a new medium, like water or glass, at an angle. This bending is called refraction. It happens because the wave changes speed. One side of the wave slows down or speeds up before the other, causing it to turn. This is why a straw in water looks broken.

When waves encounter an opening or the edge of an obstacle, they spread out instead of traveling in a straight line. This spreading is called diffraction. It allows you to hear sound from another room through a doorway even when you cannot see the source. This connects to reflection, as the diffracted waves then bounce off walls.

When two waves meet with their crests aligned, their energies add together in a process called constructive interference. This creates a single, larger wave for a moment. The new wave's amplitude is the sum of the two original amplitudes. This is one type of interference, where waves interact with each other.

When the crest of one wave meets the trough of another, their energies subtract. This is destructive interference. The resulting wave is smaller, and if the original waves are identical, they can cancel out completely for a moment. This is the opposite of constructive interference and explains why music can sound softer in some spots.