Waves Flashcards
What happens when waves travel through medium
- particles of medium oscillate + transfer energy between each other
- particles stay in place
Types of wave
- transverse
- longitudinal
Transverse waves
Oscillations perpendicular to direction of energy transfer
Example of transverse wave
Ripples on water surface
Longitudinal waves
Oscillations parallel to direction of energy transer
Oscillations
When a particle moves up/down or side to side undisturbed back and forth from rest
Compressions in longitudinal waves
Regions of high pressure due to particles being close together
Rarefactions in longitudinal waves
Regions of low pressure due to particles being spread further apart
Only type of longitudinal wave
Sound
Mechanical waves
Waves that can only travel through a medium
Electromagnetic waves
Waves that can travel through a vacuum or medium
Amplitude
Maximum displacement of a point on a wave away from its undisturbed position
What does amplitude affect
Wave intensity
Unit of amplitude
Metres (m)
Wavelength
Distance from a point on one wave to equivalent point on adjacent wave
Unit of wavelength
Metres (m)
Frequency
Number of waves passing a point each second
What does frequency affect
- pitch in sound
- colour in light
Unit of frequency
Hertz (Hz)
What is a hertz
1 wave passing a certain point per second
Period
Amount of time it takes for a full cycle of a wave
Unit of period
Second (s)
Wave speed
Speed at which energy is transferred through a medium
Practical - measuring speed of sound waves in air
- attach speaker to signal generator - generates sounds with specific frequency
- connect 2 microphones to oscilloscope so waves are detected separately
- put microphones next to speaker - slowly move 1 away until exactly 1 wavelength apart
- wavelength - distance between microphones
- frequency - what you set generator to
Expected results of measuring speed of sound waves in air
About 330 m/s
Practical - measuring speed of ripples on a water surface
- set up beater just touching surface of water
- set vibration generator to low frequency to get steady wave pattern
- put ruler at 90° to ripple tank, measure length 10 waves cover, divide by 10 for 1 wavelength
- wave speed = wavelength x frequency
How to find wave speed of piece of string
- turn on signal generator + vibration inducer to vibrate string
- adjust frequency until clear wave on string (frequency will depend on string less between pulley and transducer + masses used)
- measure wavelength of as many full waves as possible, divide for mean
- wave speed = wavelength x frequency (of signal generator)
Electromagnetic waves
Transverse waves that transfer energy from the source of waves to absorber
What do EM waves form
Continuous spectrum
What is same of all EM waves
All travel at same velocity through a vacuum or air
How are EM waves grouped
- long to short wavelength
- low to high frequency
EM spectrum
- radio waves
- microwaves
- infrared
- visible light
- ultraviolet
- x-rays
- gamma rays
What are the visible lights in order
- red
- orange
- yellow
- green
- blue
- indigo
- violet
Which EM waves can eyes detect
Visible light
Examples of energy transfer by EM waves
- campfire transfers energy to surroundings by emitting infrared radiation
- radio waves transfer energy to electrons’ kinetic stores in radio receivers, generating electric current
What can happen when wave meets a boundary
- absorbed by second material - wave transfers energy to material’s energy stores
- transmitted through second material - wave travels through material, may lead to refraction
- reflection - incoming ray sent back away from second material
What determines what happens when wave meets boundary
- wavelength of wave
- properties of material
Boundary
Interface between 2 media
What happens when wave crosses boundary
Changes speed
Normal line
Perpendicular to boundary
When is a wave at a boundary not refracted
When it is travelling along the normal line
When is a wave at a boundary refracted
When it hits the boundary at an angle
Where does the wave bend if it slows down
Towards the normal line
Where does the wave bend if it speeds up
Away from the normal line
When does a wave speed up
When it travels through a medium of a lower density
When does a wave slow down
When it travels through a higher density medium
What determines how much a wave is refracted
How much it speeds up or slows down
When does a wave speed up
When it travels through a low density medium
Optical density
- measure of how quickly light can travel though it
- higher = slower
Constructing ray diagram
- draw boundary around object
- draw normal line
- place slide in ray box
- shine ray at normal line, close to object without touching
- draw incident ray with arrows where light enters
- draw emergent ray with arrows where light exits
- measure + label angles of incidence ray + refraction + emerging ray with protractor
Which angles should be equal in refraction practical
- incident ray
- emerging ray
Wave front
Line showing all points on a wave that are in the same position as each other after given number of wavelengths
What causes waves to refract
Difference in distance travelled by wave front, caused by difference in speed
Leslie cube
Hollow, watertight metal cube with 4 vertical faces of different surfaces
Infrared practical
- empty Leslie cube on heat-proof mat
- boil water in kettle - fill cube
- hold thermometer against 4 faces - should all be same temp
- hold IR detector 10m from a face - record radiation emitted
- repeat for each face
Infrared practical expected results
- more IR radiation on black surface than white
- more on matt than shiny
- as black/matt surfaces better absorbers
What produces radio waves
Oscillating charges in electrical circuits
What is frequency of radio waves equal to
Frequency of alternating current
How to produce radio waves
- transmitter - objects charges (electrons) oscillate in to create waves
- transmitted radio waves absorbed when they reach a receiver
- energy carried by wave transferred to electrons in material of receiver - causes electrons to oscillate
- if part of complete electrical circuit, ac produced
Why is there range of frequencies in EM spectrum
Waves generated by variety of changes in atoms and their nuclei
What causes gamma rays
Change in nucleus of atom
Which types of EM waves are more dangerous
- short wavelength
- high frequency
Which are hazardous EM waves
- UV
- x-rays
- gamma
What determines effect of hazardous EM waves
- type of radiation
- size of dose
Radiation dose
Measure of risk of harm resulting from exposure of body to radiation
Danger of UV waves
- cause skin to age prematurely
- increased risk of skin cancer
Dangers of x-rays
Ionising radiation that can cause mutation of genes and cancer
Dangers of gamma rays
Ionising radiation that can cause mutation of genes and cancer
Ionising radiation
Carry enough energy to knock of electrons
Uses of radio waves
- radio communication
- analogue TV
Why are radio waves suited for radio communication
- long-wave radio wavelengths can diffract around Earth
- short-wave radio signals are reflected from ionosphere
Uses of microwaves
- satellite TV communication
- cooking food
Why are microwaves suited for satellite TV communication
- signal from transmitter transmitted into space
- picked up by satellite receiver dish orbiting thousands of km above Earth
- signal transmitted back to Earth
- received by satellite disk on ground
- slight time delay
Why are microwaves suited for cooking food
- in microwave ovens, microwaves penetrate few cm into food
- waves absorbed by water molecules in food
- water heats up
- water molecules transfer energy to rest of food molecules through heating
Uses of infrared
- electrical heaters
- cooking food
- infrared cameras
Why is infrared suited for electric heaters
- long piece of wire heats up when current flows through
- wire emits infrared radiation
- radiation absorbed by objects + air in room
- temps increase
Why is infrared suited for cooking food
Temp of food increases when it absorbs IR radiation
Why is infrared suited for infrared cameras
- given out by all objects - hotter = more
- cameras detect radiation, turns it into electrical signal - displayed on screen as picture
- brighter picture = hotter
Uses of visible light
Fibre optic communications
Why is visible light suited for fibre optic communications
- light rays reflected back and forth until they reach end of fibre
- not easily absorbed + scattered down cable
Uses of ultraviolet
- energy efficient lamps
- sun tanning
Why is ultraviolet suited for energy efficient lamps
- UV light absorbed, visible light emitted from fluorescent materials
- lamps generate UV, re-emit as visible light using layer of phosphor
Why is ultraviolet suited for sun tanning
- UV radiation produced by sun, exposure causes tan
- tanning salons - UV lamps give artificial suntan
Uses of x-rays
Medical imaging
Why are x-rays suited to medical imaging
- pass easily through flesh, not denser materials like bone
- amount of radiation absorbed gives x-ray image
Uses of gamma rays
Medical treatments
Why are gamma rays suited to medical treatments
- radiotherapy - treat cancer
- rays carefully directed towards cancer cells
- avoids killing healthy cells
