Topic 4- Waves

waves transfer___ but not___

energy but not matter

wavelength (lamda)

distance between the same points on 2 consecutive waves

Amplitude (A)

distance from equilibrium line to maximum displacement

Frequency

number of waves that pass a single point per second

Time Period (T)

time taken for a whole wave to completely pass a single point

wavefront

a way of picturing waves from above with lines

Wave equation

v = f x lamda (velocity(m/s) = frequency (Hz) x wavelength (m))

frequency and period relationship

f = 1/T

transverse wave

vibration is perpendicular to direction of wave motion

longitudinal waves

vibration parallel to direction of wave motion

longitudinal waves can’t move in a…

vacuum

close together bits of a longitudinal waves

compressions

spaced apart bits of longitudinal waves

rarefactions

wavelength of a longitudinal wave

distance between the centre of 1 compression and the next

top and bottom points of a transverse wave

peaks and troughs

symbol for wavelength

lambda

3 ways of measuring speed of sound waves

• 2 people stand >100m apart and time how long after seeing they hear something (least accurate)

• use an echo and stand >50m away from a wall, time how long after seeing they hear something

• use an oscilloscope that uses 2 microphones (most accurate)

calculate the distance (x) of something using wave speed and time

x = v x t

calculate depth of water using wave speed and time

depth = ½ x speed in water x time to receive echo

reflection

a wave hits a boundary between 2 media but doesn’t pass through and is reflected back

flat, smooth surfaces are ___ reflectors

good

rough surfaces are ___ reflectors because they ___ light

bad, scatter

which surfaces always reflect some light (some is absorbed)?

opaque

Refraction

a wave changes speed at the boundary between 2 media of different densities, sometimes causes wave to change direction

which types of waves can be refracted?

• sound

• seismic

• water

• electromagnetic

Transmission

a wave passes through a substance (must emerge other side)

what happens to a wave when it is transmitted?

• partially absorbed (usually)

• amplitude may decrease (e.g. sound waves quieter through a wall)

Absorption

energy is transferred from the wave into the particles of a substance

Why do some waves change direction when they are refracted?

different parts of the wave enter denser medium at different times, causing a difference in speed within the wave. causes it to bend.

what affects how materials interact with waves?

wavelength

measure speed of water waves by creating ripples

• choose a clam, flat water surface

• measure distance between 2 people

• 1 person disturbs water, the other times how long it takes to get to them

• repeat 10 times and find average

• calculate speed (d/t)

rarefactions and compressions create changes in…

pressure

how do sound waves cause vibrations in solids?

fluctuating pressure of waves hits solids and causes particles to vibrate

natural frequency of a solid

the frequency a solid tends to vibrate at

how does natural frequency change affect of sounds waves on solids?

• sound waves with frequency close to natural frequency cause larger vibrations compared to sound waves with much higher or lower frequencies (to natural frequency)

• therefore some frequencies are transferred more efficiently to the solid

why can sound waves be heard by human ears

they are transferred efficiently to solid components of ear

why does transmission of sound to human ear only work in a limited frequency range?

sound waves outside the frequency range are too far from natural frequency of ear components

range of human hearing

20 Hz - 20,000 Hz

How the ear works

• sound waves travel down auditory canal

• pressure variations exert varying force causing eardrum to vibrate

• vibrations transferred to 3 small bones that amplifies them

• vibrations transferred to liquid in cochlea in inner ear

• tiny hairs in cochlea detect vibrations create electrical impulses which travel to brain and are interpreted as sound

ultrasound

sounds waves with frequency over 20,000 Hz

Infrasound

sound waves with frequency under 20 Hz

uses of ultrasound in sonar

• detects objects underwater

• sound wave reflected off ocean floor

• time it takes for sound wave to return used to calculate depth of water (unusual variations in depth = object underwater)

uses of ultrasound in foetal scanning

• used to construct images of foetus in womb

• transducer produces beam of ultrasound into womb

• different boundaries between tissues reflect ultrasound back to transducer

• electrical signals are sent when echoes are detected, depths and distances calculated

• computer builds up an image

• non-invasive and harmless

P waves

• Primary waves- faster than S waves so felt first in an earthquake

• longitudinal infrasound waves caused by earthquakes

• can pass through solid and liquid layers of earth

• refracted as they pass through layers, causing ‘shadow zones’ where they are not detected

S waves

• Secondary waves- slower than P waves so felt 2nd in an earthquake

• transverse waves caused by earthquakes

• only travels through solids

• can’t pass through liquid outer core, so not detected on opposite side of earth’s surface to quake

why seismic waves are important to understand structure of Earth

interior of Earth not physically possible to observe so provide evidence about structure

conclusion from discovery that only P-waves are detected on opposite side of Earth to quake

• Mantle is solid- both waves can pass through

• outer core is liquid- s waves can’t pass through it

why do refractions between layers in the Earth cause 2 shadow zones?

inner core is solid, so large refraction is taking place

what can change about a wave from one medium to another?

• wavelength

• velocity

• frequency

These are all linked by the wave equation, meaning changes in one causes changes in others

sound travels ___ in hot air

faster