20-1 The Nature of Waves

What is a wave?

• Waves are disturbances that transmit energy through matter or empty space– Discuss with a neighbor the different kinds

of waves taking place in the room right now

• Light waves, sound waves, water waves, waves in the flag, etc…

Wave energy• Waves carry energy

away from its source– ie: dropping a rock in a

pond or moving a slinky– the material that the wave

moves throuhgh does not move though, only the energy passing through it moves

Energy transfer through a medium• Most waves transfer energy by

vibrating the particles in a medium– A substance through which a

wave can travel• ie: water, air, etc…

– Can be solid, liquid or gas– Discuss with a neighbor why

waves transfer energy through solids faster than through gases (think back to chem)

• Because the particles of a solid are closer together than those of a gas so they can make each other vibrate faster because they bump into each other more often

• Waves that need a medium to travel are called mechanical waves• Examples: sound waves and water waves

Energy transfer without a medium

• Some waves do not need a medium to transfer energy – they can travel through empty space (a vaccuum)– ie: visible light, microwaves from a microwave oven, tv and

radio waves (signals) x-rays– These are all electromagnetic waves– Although they don’t need a medium to be transmitted, they

can go through matter• Find a neighbor and discuss why it is important that electromagnetic

waves can travel through a vaccuum (think about visible light/the sun)

• Because space is a vacuum – there is no matter in space – so if EM waves could not travel through empty space, light could not travel from the sun to the Earth and we would be in total darkness

Types of waves

• All waves transfer energy through vibrations

• Waves can be classified by the direction the particles of the medium vibrate compared to the movement of the wave

• Two main types of waves:– Transverse and longitudinal

Transverse waves• Waves that cause medium particles to vibrate up and

down while the wave travels left to right– Medium particles move perpendicularly to the direction that the

wave is moving

Crest

• A wave’s amplitude is related to its height– It is the max. distance

the particles of a medium vibrate from their rest position

• The rest position is where the particles are when there is no disturbance

– The green arrows show the amplitude of this wave

– The larger the amplitude, the taller the wave and the more energy passing through the wave

Parts of a wave• The crest of a transverse wave is found at

the highest point of the wave

• The trough of a transverse wave is found at the lowest point of the wave

Crest

trough

Longitudinal waves• The particles of the medium vibrate back and forth along

the path of the wave movement– They travel parallel to each other – in the same direction

Parts of a wave part II• The compressions of a longitudinal wave

are found where the medium’s particles are closest together

• The rarefactions of a longitudinal wave are found where the medium’s particles are farthest apart

• The wavelength of a wave is the distance between any two crests or compressions next to each other– It can also be measured between two troughs or

rarefactions

• A shorter wavelength carries more energy than a longer wavelength