waves

• types of waves

  waves transfer energy, they do not transfer matter
• longitudinal waves

  are waves whose oscialtions are parallel to the direction of wave travel

  eg: sound waves

  the wavelength is a measure between the rarefractions or between the compressions

  longitudinal waves are mechanical meaning they require a medium to travel through (sound through air)

  
• transverse waves

  are waves whose oscillations are perpendicular to the direction of waves travel

  eg: light waves

  the wavelength can be measured at any point, but is usually from peak or crest

  electromagnetic waves are transverse waves and are caused by oscillations in the electric and magnetic field
• features of a wave

  • equilibrium: the undisturbed positions the particles are in when they are not oscillating
  • displacement: distance moved from equilibrium
  • peak: highest point of a wave
  • trough: lowest point of a wave
  • amplitude: maximum displacement from displacement (longest distance from eqilibrium to peak or trough)
  • wavelength: distance covered from full cycle of a wave
  • time period: the amount of time taken for a full wave cycle (usually peak to peak or trough to trough)
  • frequency: the number of waves passing a set point per second (measured in Hertz)
• wave speed and distance

  wave distance is the wavelength, and if the directon of the wave is labelled in distance units then the wavelength is distance between wave cycle

  the period is the wavelength, when the direction is time, it is the amount of time it takes to get from one point to the next corresponding point

  A) distance

  B) time

  
• the time period of a wave can be calculated using

  time period(sec) = 1/frequency(hz)
• wavespeed can be calculated using

  wavespeed(m/s) = frequency(Hz) x wavelength(m)
• the air is made up of particles, and sound is caused by oscillating and colliding air particles

  the vibrating particles pass the sound to a persons ear and vibrates the ear drum

  sound can be measured practically and calculated with

  speed(m/s) = distance(m) / time(s)

  the speed of sound through air is 330m/s
• measuring waves - ripple tanks

  a ripple tank is a shallow tank of water which vibrates (because of a vibrating bar, connected to a power supply)

  above it we have a lamp and below the tank there is a sheet of paper

  we record the waves - gives us the power to pause and slow down the video

  1. place a ruler next to the paper (measuring wavelength)
  2. measure 10 wavelengths
  3. divide by 10 to get individual length of wave

  for the frequency

  1. place a timer next to the paper
  2. count the number of waves passing a point in 10 seconds
  3. divide by 10 to get waves per 1 second (which is frequency)
• with the ripple tank we can measure wavespeed, wavelength and frequency from the measurements
• setup for measuring a wave in a solid practical

  A) string

  B) mass

  
• measuring waves - in a solid

  a string is attached to a vibration generator, which is connected to a signal generator

  we need the mass to keep the string taut

  the signal generator allows us to change the frequency of the vibrations on the string

  1. measure the total length of the string (with all the waves, just the whole thing)
  2. divide this by the number of half wavelengths - the waves are overlapped

  the wavelength depends on the tautness of the strinn and the mass of the weight
• we can use the measurements from measuring waves in a solid to calculate wavelength and wavespeed. for wavespeed we get the frequency values from the signal generator
• electromagnetic waves

  oscillations in an electromagnetic wave are caused by changes in the electric and magnetic field at right angles, in the direction of travel

  all electromagnetic waves:

  • transfer energy as radiation (from the radiation source to the absorber)
  • can travel through a vacuum ---> space
  • travel at the same speed through air or a vacuum - the speed of light
• electromagnetic waves

  electromagnetic waves form a continuous spectrum of waves, including:

  • waves with short wavelengths: they have high frequency and high energy
  • waves with long wavelengths: they have low frequency and low energy

  the electromagnetic spectrum goes:

  radio waves --- micro waves --- infrared --- visible light --- ultra violet --- x rays ---> gamma rays

  (in order from longest to shortest wavelength)

  Red Martians Invade Venus Using X ray Guns
• electromagnetic waves

  each group contains their own range of freqencies

  eg: visible light contains red (lowest frequency) and purple (longest wavelength)

  remember: there is a corrolation between wavelength and frequency, if you have a small wavelength, more wavels are available to pass a point per second
• the behaviour of an electromagnetic wave depends on its frequency
• uses of radio waves

  radio waves are transmitted easily through air

  the do not cause damage to the body if absorbed and can be reflected to change their direction

  > ideal for communication

  radio waves can be produced by oscillations in a current

  when absorbed by a conductor they create an alternating current (an electric current that regularly changes its direction and size)

  this electrical current has the same frequency as the radio waves

  information is encoded on the wave before transmission, and decoded when recieved

  > television and radio use this to broadcast information
• microwaves

  high frequency microwaves have frequencies that are easily absorbed by food molecules

  the internal energy of the molecules increases when absorbed by microwaves ---> heating

  they also pass through the atmosphere easily

  > can pass between stations on earth and satellites

  used for heating food and for satellite communication
• infrared

  infrared light has frequencies which are absorbed by some chemical bonds

  the internal energy of these bonds increases when they absorb infrared light ---> heating

  > infrared is used for cookering and electrical heaters

  all objects emit infrared light

  the human eye cannot see this but infrared cameras can

  > thermal imaging (used to detect people in the dark)
• visible light

  used in fiber optic communication

  coded pulses of light travel through glass fibres - from a source to a reciever
• behaviour of ultraviolet waves

  we cannot see it but it has hazardous effects on us

  > can cause the skin to tan or burn

  fluorescent substances absorb ultraviolet light (produced inside the lamp) and re-emit the energy as visible light

  > ultraviolet light is used in the form of energy efficient light bulbs
• electromagnetic waves in medicine

  changes in atoms and their nuclei can cause electromagnetic waves to be generated or absorbed

  gamma rays are produced by changes in the nucleus

  high energy waves such as gamma and x - rays are transmitted through the body with little absorption

  > ideal for internal imaging

  x-rays are absorbed by dense structures like bones (why they're used to find broken bones)
• ionising radiation

  ultraviolet waves, x-rays and gamma rays are types of ionising radiation

  > they can add or remove electrons from molecules producing electrically charged ions

  it can have hazardous effects on the body

  • ultraviolet waves causes skin to age prematurely and increase risk of cancer
  • x-rays and gamma rays can cause gene mutations, which can lead to cancer
• radiation dose

  its a measure of the risk of harm caused by exposing the body to ionising radiation

  radiation does is measured by Sieverts (Sv)

  the figures are usually in small amounts and are commonly measured in millisieverts (mSv)

  1Sv = 1000mSv
• background radiation surrounds us all the time, including:

  • radioactive rocks in the earth's crust
  • cosmic rays
  • man made sources - nucleur weapon fallout and nucleur accidents

  the level of background radiation and dose are effected by factors, such as the jobs that people do and where they live
• waves (including sound and light) can be reflected at the boundary of two different material

  a relection of sound is an echo
• the law of reflection states that:

  the angle of incidence = the angle of reflection

  the angle of incidence and refraction are measured between the light ray and the normal

  normal: an imaginary line at right angles between the boundaries, all angles are measured to this line
• this diagram shows the angle and ray of incidence and reflection

  A) reflected

  B) incident

  C) normal

  
• specular reflection

  specular reflection: when light is travelling towards a surface in one direction, and is reflected in a single direction

  this happens from a smooth, flat surface

  the image (including in an mirror) produced is:

  • upright
  • virtual

  virtual image: rays appear to diverge from behind the mirror

  > the image looks like it is coming from behind the mirror
• diffuse reflection

  diffuse reflection: when light is reflected off a rough surface and scattered in different directions

  > causes a distorted image (like ripples on water) or no image

  each individual ray obeys the law of reflection, but the surface are at different angles

  
• refraction

  different materials have different densities, and when light travels through the boundary of two transparent materials, it can change direction

  A) normal

  B) refracted ray

  
• refraction

  the density of a material will change the speed that a wave is transmitted through it

  > the more denser a transparent material, the slower light travels through it

  glass is denser than air

  a light ray travelling from air into glass will slow down, and bend towards the normal

  the reverse is also true: light travelling from glass to air will speed up, and bend away from the normal

  A) angel of incidence

  B) angle of refraction

  
• if a wave slows down, the wavelength will decrease (because there are less crescents to pass per second)

  as a wave travels into a denser medium (ie: water) the wave slows down and the wavelength decreases ---> the distance decreases

  the frequency remains the same (because of the change in wavelength)
• required practical - infrared absorption and emission

  we use a leslie cube to see how much infrared is emitted from different surfaces

  our surfaces: shiny metallic, white, shiny black and matte black

  1. fill leslie cube with hot water
  2. point an infrared detector at each of the four surfaces and record the amount of infrared emitted
  3. keep the distance between the detector and the cube the same! (this makes the practical repeatable)
  4. if we don't have a detector we can use a thermometer with a black painted bulb (lower resolution though)
• required practical - infrared absorption and emission

  another method/version

  1. place a drawing pin onto each face, attach it with vaseline
  2. turn the heater on
  3. record the time taken for vaseline to melt and the pin to fall off
• infrared absorption always occurs

  an object will always emit and absorb infrared radiation

  all objects obey this rule

  • when an object absorbs infrared it will get warmer
  • if an object emits infrared it will get cooler
  • black surfaces absorb all visible light
  • a good absorber of infrared is also a good emitter of infrared
• the temperature of a body is linked to the balance between the amount of energy it absorbs and emits

  if the rate of absoption is greater than the rate of emission (absorbing more than it is emitting): the temperature increases

  if the rate of absorption is equal to the rate of emission (absorbing and emitting the same amount): the temperature is constant

  if the rate of absorption is less than the rate of emission (it emits more than it is absorbing): the temperature is decreasing
• the temperature of earth depends on

  • concentration of greenhouse gases (carbon dioxide, water, methane)
  • the rate infrared is absorbed by the atmosphere
  • the rate infrared is emitted by the atmosphere
• earth's temperature

  visible light and high frequency radiation is able to enter the earths atmosphere because they have a short enough wavelength to penetrate the atmosphere

  when they are absorbed by the earth the planet's internal energy increases ---> surface gets hotter

  some of this energy is transferred to the atmosphere by convection and conduction