Introducing waves

Demonstration

The point of these questions is to stimulate curiosity and to demonstrate that waves have some interesting properties. At an introductory stage, you must leave many questions unanswered.

Apparatus and Materials

• Sound sources, such as tin whistles, guitar, radio
• Small pieces of closely-woven fabric for each student
• Lamp (e.g compact light source, 12V
• Low voltage power supplies, 2 (compact light source will need 8A low voltage power unit)
• Radiant heater, low voltage

Health & Safety and Technical Notes

Do not use a mains-powered electric fire with a bare element. Use either a low-voltage infra-red source or (better) a mains-powered one with the element(s) inside silica tubes.

Read our standard health & safety guidance

Procedure

Run through a series of questions and demonstrations about various wave phenomena, for example:

Sound: echo and speed

• Say: "When you talk, sound waves carry your message through the air to other people. Clap your hands and listen for the echo."
• Ask: "How do sounds travel so fast? Can sound waves in air travel faster than the wind?"
• Ask: "How do you know that sound carries energy?"

Sound: beats

• Play two sources of sound of slightly different frequencies, using whistles, small organ pipes, monochords, or tuning forks.
• Say: "The throbbing sound is called 'beats'"
• Ask: "How do two sounds create these beats?"

Waves at sea

• Ask: "How do waves travel across an ocean from a remote storm?"
• Ask: "How do you know that ocean waves carry energy?"
• Ask: "Have you noticed that wave crests get closer together as they roll up a beach? That they change direction as they round a head, or enter narrow gates of a harbour?"

Radio

• Say: "The signals of radio and television come to us as waves."
• Ask: "What carries them?"
• Ask: "When you hear a time signal on the radio (or catch a programme on TV), do the signals arrive noticeably later if you are far away from the broadcasting station?"
• Say: "Those signals bring in a tiny stream of energy that triggers a lot more energy from the mains or batteries."

Light

• Give each student a small piece of closely-woven cloth. Get students to look at the lamp through the cloth. Also, suggest looking at a distant street lamp at night through an umbrella.
• Ask: "Why do you see an extra pattern of bright spots?"
• For comparison, set up a funnel over the table and put fine dry sand in it. Hold a fine-mesh strainer in the stream of sand."
• Ask: "does the sand make an extra pattern of heaps when it goes through a mesh or just a single pile?"
• Ask: "What makes the difference between light and sand patterns?"

Radiation, visible and invisible: Energy

• We call the colours of visible light, infra-red and ultra-violet, and so on ‘radiation’. They all carry energy.
• Ask: "What do you feel when you hold your hand near a glowing fire or an electric heater?"
• Ask: "What happens if someone walks between you and the heater? Can you still feel its warmth?"

Teaching Notes

• The aim of this introductory discussion is to whet students' appetite for learning more about waves. Waves are all around us and behave in interesting ways. You could answer students' questions with further questions.
• The explanations given below are for teachers only. Students' understanding should develop gradually, later, as they try further experiments.
• Sound echo and speed: Sound can travel faster then the wind. The waves are carried by collisions between molecules in air transferring movement energy. A sound wave is made up of pressures that are alternately higher (at compressions) and lower (at rarefactions). There are many ways that you can tell sound carries energy, e.g. it pushes eardrums in-and-out.
• Sound beats: Waves are unique in their ability to simultaneously pass through any point without permanently affecting each other. But the effect at a point is a superposition of the effects of the waves that are passing. This is called interference. The way that two sounds of slightly different frequency interfere produces beats. Musicians use this effect in tuning their instruments.
• Waves at sea: Sea waves are carried in a different way than sound waves. There is an attractive force between water molecules - when one molecule moves, it makes its neighbour move too. You can see that waves carry energy because waves move stones on a beach, boats or people in the water. The change in spacing and direction of wave peaks along a shore shows that the wave speed decreases as the water gets shallower.
• Radio: Radio waves are a form of electromagnetic wave. There is no noticeable time delay because the speed of these waves is the speed of light, extremely fast.
• Light: When they look through the cloth, students will see a diffraction pattern. Only waves, and not particles, make diffraction patterns. This is evidence that light is a wave.
• Radiation, visible and invisible: To be more precise, this is called electromagnetic radiation. A radiant heater produces infrared light. It behaves just like visible light, and opaque objects (e.g. the person who walks between you and the heater) will cause shadows.

This experiment was safety-tested in February 2007