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School-college Physics Notes: Waves 1. What are waves?

Introduction to waves: 1. Introduction to waves - what are they? and types of waves?

Doc Brown's Physics exam study revision notes

1. Introduction to waves - what are they? and types of waves?

Be able to describe that waves transfer energy and information without transferring matter.

Waves are vibrations of a material e.g. sound or oscillations of an electromagnetic field e.g. visible light.

With transverse waves the oscillations are at right angles to the direction the wave is travelling.

A transverse wave has an 'up and down' component in its description.

With longitudinal waves the oscillations are in the same direction as the wave is travelling.

A longitudinal wave has an 'to and fro' component in its description.

Waves have energy and can transfer it from one place to another.

This means waves can carry information by changing the energy of the wave in a sequential way to transfer 'data' from one place to another, but waves cannot transfer the matter of the medium they are moving though - the medium can be gas, liquid or solid.

Electromagnetic radiation does not need a medium, it can travel through vacuum as well as transparent gases (e.g. air), liquids (e.g. water) or solids (e.g. glass).

Infrared and visible light beams in fibre optic cables can carry enormous amounts of information often as telephone calls or the networks of the internet - the information is converted to binary code and the 'pulsed code' is imposed on, and transferred by, the visible or infrared carrier wave.

When the signals reach your mobile phone or computer, they are decoded and presented in an audio-visual way.

When your TV receives the signal, its just coded data in the electromagnetic wave, energy is transferred, but no material substance arrives!

However, if energy itself wasn't transmitted, no effect could be produced on the TV screen without something from the receiver!

Similarly, when ripples on water cause floating objects to bob up and down, energy is needed to do this, but neither the floating object or the water itself actually move in the direction of the transverse waves - but energy is transferred from one location to another.

The most dramatic transfer of energy involves seismic earthquake waves, both transverse and longitudinal, yet the effects are transmitted and felt miles from the epicentre and no part of the earth's crust moves in the direction of the seismic waves but it may move violently from side to side, up and down or compressed/decompressed.

When sound waves vibrate your ear drum no air moves from the TV, person or musical instrument to your ear, yet energy is transferred, carried by vibrations, through the medium of air, otherwise, what could cause your ear drum to vibrate!

You need to know the two formulae for waves relating them to wavelength, frequency, speed and distance travelled and calculations and problem solving based on them.

speed = distance / time  and  speed = wavelength x frequency

Know the differences between longitudinal and transverse waves by referring to sound, electromagnetic and seismic waves.

You need to understand that in a transverse wave the oscillations are perpendicular (at 90o) to the direction of energy transfer, but in a longitudinal wave the oscillations are parallel to the direction of energy transfer ie direction of forward wave movement.

Transverse wave diagram

Shaking a slinky spring from side to side (at 90o to stretched out spring) in a regular rhythm produces a transverse wave of energy pulses.

Similarly wave ripples on water and all electromagnetic radiation waves oscillate at 90o to the direction the wave is travelling.

Longitudinal wave diagram

Pulling and pushing on a slinky spring in a regular rhythm produces pulses of energy transmitted as a longitudinal wave down the stretched out spring.

Similarly a sound wave is pulses of energy travelling through a medium - the 'compressions' and 'rarefactions' are in the same direction as the wave movement.

More on examples of waves in the part 2

Check out your practical work you did or teacher demonstrations you observed, all of this is part of good revision for your end of course examination context questions and helps with 'how science works'.

using a class set of skipping ropes to investigate frequency and wavelength,

demonstrating transverse and longitudinal waves with a slinky spring,

carrying out investigations using ripple tanks, including the relationship between depth of water and speed of wave,

Keywords, phrases and learning objectives for waves

Know, and be able to explain what waves are and describe the two types of longitudinal waves and transverse waves in terms of the direction of oscillations-vibrations and the fact that they transfer energy.

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