waves
transverse and longitudinal waves
waves transfer energy in the direction they are travelling
when waves travel through a medium the particles of the medium oscillate and transfer energy between each other-gut overall particles stay in same place only energy is transfered
amplitude= maximum displacement of a point on a wave from its undisturbed position
wavelength= distance between same point of 2 adjacent waves
frequency= number of complete waves passing a certain point per second (Hz)
period= 1 / frequency
all waves are transverse or longitudinal
transverse
oscillations are perpendicular to the direction of energy transfer
e.g. all electromagnetic waves, ripples in water, a wave on a string
longitundinal
the oscillations are parallel to direction of energy transfer
e.g. sound waves, seismic waves
wave speed= frequency x wavelength
experiments with waves
use an oscilloscope to measure speed of sound
1) set up the oscilloscope so the detected waves at each microphone are shown as separate waves
2)start with both microphones next to speaker, then slowly move one away until both waves are aligned on the display, but have moved exactly one wavelength apart
3) measure the distance between the microphones to find one wavelength
4) use v=f x y to calculate the speed
measure the speed of ripples using a lamp
1) use signal generator attached the dipper of the ripple tank you can create waves at set frequency
2) use lamp to see waves crests on screen below tank
3) the distance between each shadow line is equal to one wavelength . measure the distance between shadow lines that are 10 wavelengths apart then divide by 10 to find average wavelength
4) use v= f x y to find wave speed
waves on strings
1) set up equipment then turn on signal generator and vibration deducer
2) adjust frequency on signal generator until there is a clear wave on the string -frequency needed depends on masses used, length of string between pulley and deducer
3) measure wavelength of these waves
4) the frequency of the wave is whatever signal generator is set to
5) find speed using v= f x y
reflection
all waves are absorbed transmitted or reflected
when waves arrive at a boundary 3 things might happen:
-waves are absorbed by material; this transfers energy to materials energy stores
-the waves are transmitted; keep on travelling through
-waves are reflected
ray diagram
angle of incidence= angle of refection
specular or diffuse
waves are reflected at different boundaries in different ways
specular= wave is reflected in a single direction by a smooth surface
diffuse= when a wave is reflected by a rough surface and are scattered in different directions
diffuse is caused by normal being different for each ray so angle of incidence and reflection are different for each ray
electromagnetic waves and refraction
continuous spectrum of em waves
all em waves are transverse that transfer energy from source to absorber
all em waves travel at same speed through air or vacuum
large range of frequencies because they are generated by a variety of changes in atoms and their nuclei
coz of different properties, em waves are used for different puroses
refraction
= when a wave crosses a boundary between materials at angle and changes direction
how much the wave is refracted by depends on wave speed - density of new medium
if a wave crosses boundary and slows down it will bend towards normal
frequency stays the time
if wave is travelling along normal it will change speed but is not refracted
optical density= measure of how quickly light can travel through it- higher op, slower light travels through it
investigating light
need to do experiments in dim room
use transparent materials to investigate refraction
1) place transparent block on piece of paper and trace around it. use a ray box to shine a at of light at middle of one side of block
2) trace incident ray and mark where light emerges on other side of block. remove block and draw a straight line joining up rays to show refracted ray through block
3) draw normal where light entered block, use protractor to measure angle of incidence and angle of refraction
4) repeat using different materials with same incident ray
different materials reflect light by different amounts
1) take paper and draw straight line across it and place object so one side lines up with line
2) shine ray of light on objects surface and trace incoming and reflected rays
3) draw normal at point ray hits object use protractor to measure angles and I and R - note width and brightness of ray
4)repeat for different objects
radio waves
made by oscillating charges
em waves are made up of oscillating electric and magnetic fields
alternating currents are made up of oscillating charges- as charges oscillate they produce electric and magnetic field i.e. em waves
frequency of waves produced will be equal to frequency of ac current
the frequency of the waves produced are equal to the frequency of alternating current
radio waves are produced by using an ac current in an electrical circuit. the object in which charges (electrons) oscillate to create the radio waves is caused a transmitter
when transmitted radio waves reach a receiver they are absorbed
the energy carried by the waves is transferred to the electrons in the material of the receiver
this energy causes electrons to oscillate and if the receiver is part is part of a complete electrical circuit, it generates an ac current
the current has same frequency as the radio wave that generated it
used mainly for communication
radio waves are em radiation with wavelengths longer than about 10cm
long wave radio wavelengths (1-10km) can be transmitted around world because they diffract around the curved surface of the earth and hills, tunnels, etc
this makes it possible for radio signals to be received even if it is not in line of sight of transmitter
short wave signals can be received at long distances because they are reflected on ionosphere- an electrically charged layer in the earths upper atmosphere
Bluetooth use short waves to send signals over short distances between devices without wires
medium wave signals can also reflect on ionosphere depending on conditions and time of day
em waves and their uses
microwaves
used for satellite communications
pass easily through atmosphere
for satellite tv the signal from transmitter is transmitted into space where it is picked up by satellite receiving dish which then transmits the signal back to earth in a different direction. where it is recivfeved by a stalite dish on the ground - slight delay due to long distances
microwave ovens
microwaves need to be absorbed by water molecules in food- different wavelength to communications
waves penetrate few cm of food before being absorbed and transferring energy to water molecules in food, causing it to heat up
water molecules transfer this energy to rest of food by hitting- quickly cooks food
infrared radiation
given out by hot objects, hotter the object the more it gives out
infrared cameras monitor temp, camera turns IR into electrical signal which is displayed as a picture - hotter object, brighter it is on screen
electrical heaters
contains long piece of wire which heats up when current passes through it which emits IR - the IR is absorbed by objects whose heat energy stores increase
more uses of em waves
visible light
fibre optic cables contain thin optical wires (plastic) that carry data over long distance pulses of visible light
work through reflection- light rays are bounced until reach end of fibre
not easily absorbed or scattered
ultraviolet
fluorescence is uv in some chemicals causing them to glow, radiation is absorbed, light is emitted
fluorescent lights generate uvr, absorbed and reemitted as visible light by a layer of phosphorus- energy efficent
security pens- under uv light the writing glows, otherwise invisible
uv lamps in tanning salons give artificial suntan
xrays
xray photographs show images of broken bones
xrays pass through felsh not bone, amount of radiation absorbed gives image
can be used in cancer treatment to kill cells
gamma
medical tracers - gamma source is injected and its progress around body is followed, can pass out of body to be detected
dangers of em waves
harmful to people
low frequency waves do not have much energy, not absorbed
high frequency (uv) transfer lots of energy, cause damage
uv- damages surface cells (sunburn). xrays/ gamma (ionizing radiation)- gene mutation, cancer
sieverts
radiation dose is measured in Sieverts which measures risk of harm being exposed to radiation
people look at whether benefits outweigh health risks
not measure of total amount of radiation absorbed
1000mSv=1Sv
risk depends on amount of radiation absorbed and how harmful type is
risk can be different for different parts of the body
lenses
lenses produce different kinds of image
convex
bulge outwards
causes rays of light to converge at principal focus
principal focus is where rays hitting less parallel to axis meet
concave
caves inwards
causes parallel rays of light to diverge
principal focus is where rays hitting the lens parallel to the axis appear to come from
distance from centre of lens to principal focus= focal length
rules for rarefaction
convex
incident ray parallel to axis refracts through lens and passes through principal focus
incident ray passing through centre of lens carries on in same direction
concave
incident ray parallel to axis refracts through lens and travels in line with principal of focus
incident ray passing through centre of lens carries on in same direction
images and ray diagrams
real or virtual
real image= where light from object comes together to form an image on screen ( eyes retina)
virtual= when rays are diverging so light from object appears to come from a completely different place (magnifying glass)
size, upright or inverted, real or virtual
concave lenses and magnification
concave lenses always create virtual images
magnifying glass
use convex lenses
object being magnified closer to lens than focal length
virtual- light rays don't actually come from where image appears
magnification= image height / object height
visible light
made up of range of colours
violet (400nm)->red(700nm)
primary colours= red, green, blue
all these different colours together create white light
colour and transparency depend on wavelength
different objects absorb, emit, reflect different wavelengths
opaque= objects do not transmit light (absorb some wl's and absorb others)
colour of opaque object depends on which wl's are most strongly reflected
for objects that are not a primary colour they may be reflecting wavelengths of light corresponding to that colour or the wl's of primary colours are mixed toegther
white objects reflect all wl's equally
black objects absorb all wl's of visible light
transparent/ translucent objects transmit light- some pass through
some wl's may be absorbed or reflected by transparent objects- objects colour is related to wl's absorbed/ reflected
colour filters
filter out different wl's so that only certain colours are transmitted
a primary colour filter only transmits that colour
filters that aren't for primary colours let through both wavelengths of light for that colour and the wavelengths of the primary colours that can be added together to make that colour
red object through blue filter appears black
infrared radiation and temp
every object emits and absorbs IR
all objects are continually emitting and absorbing infrared radiation
the hotter an object is, the more it emits
an object that's hotter than its surrounding emits more IR than it absorbs as it cools down
objects at constant temp, absorb and emit IR at same rate
some colours and surfaces emit and absorb radiation better than others (matte, black-leslies cube)
leslies cube
4 vertical faces have different surfaces
1) place cube on mat and fill with boiling water
2) hold thermometer against each side of cube
3)hold infrared detector at set distance and record amount of IR radiation it detects
4) repeat for each side
5) black, matte surface emits and absorbs the most
black body radiation
ultimate emitters
perfect black body= object that absorbs all radiation that hits it
all objects emit em waves due to heat energy stores but perfect black body are the best possible emitters
the intensity and distribution of wl's emitted depends on objects temp
as temp of object increases, intensity of each wl emitted increases
intensity increases more rapidly for shorter wl's -causes peak wl to decrease
radiation effects earths temp
overall temp of the earth depends on the amount of radiation it emits, reflects and absorbs
in the day lots of radiation transferred to the earth by the sun in absorbed, causes increase in local temp
at night, less radiation is being absorbed than emitted, decrease in temp
overall temp of earth stays fairly constant
changes to atmosphere can change earths overall temp, if atmosphere starts to absorb more radiation without emitting same amount the overall temp will rise until they are equal (global warming)
sound waves
caused by vibrating objects- vibrations are passed through the surrounding of the medium as a series of compressions and rarefactions
longitudinal
sound travels in a solid by particles in the solid vibrating
cant travel through vacuum
eardrum vibrates
sound waves reach eardrum causing it to vibrate
passed on to tiny bones called ossicles through semicircular canals and to cochlea
cochlea turns vibrations into electrical signals
human hearing is limited by size and shape of the eardrum as well as the structure of the parts of the ear that vibrate
reflect or rarefract
sound can be reflected by hard flat surfaces (echoes)
sound waves refract when they enter a different medium, wl changes but frequency stays the same so speed also changes