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iGCSE Physics - Waves (Sound (Types of Waves Sound is sound wave…

- - - - It needs a material to travel in
      - A sound wave has a source and spreads out from the source
      - Sound waves arise because of the vibration of a source
    - - The regions of high pressure are called compressions
      - The regions of low pressure are called rarefaction
  - - - In air, the speed of sound is considered to be about 340 m/s
      - In water, the speed of sound is considered to be about 1500 m/s
      - In steel, the speed of sound is considered to be 6000 m/s
    - - Wave speed = frequency x wavelength
  - - - The human ear is most sensitive to 3000 Hz, the frequency of a human scream
        
        Babies cry and alarms sound at this frequency
        
        3000 Hz sounds are very penetrating and will make you feel very uncomfortable
      - As you get older, the upper limit of hearing goes down
    - - Ultrasound (above 20KHz)
        
        This is used by bats for navigation by SONAR (sound navigation and ranging). It is also used by a non-destructive investigation equipment.
      - Infrasound (below 20Hz)
        
        This is often used by very large animals (e.g. elephants) for communication over long distances
  - - - The microphone turns the sound waves into electrical waves, which the CRO displays on screen
      - There are three main properties that we can display on the screen
        
        The frequency (pitch) - number of waves per second
        
        Amplitude (volume) - the size of the waves
        
        Quality (timbre) - shape of the waves
  - - - The bigger the wave, the louder the sound
  - - - The more waves shown, the higher the frequency
  - - - A ship using SONAR sends pulses of high frequency sound to the bottom of the sea. It has microphones that pick up echoes, and the depth of the water can be worked out from the time taken between sending the pulse and receiving the echo.
- - - - c = 300 000 000m/s = 3 x 10^8m/s
    - - The boundaries between the different forms of light are not distinct. ie. waves of wavelength 0.01 nm may be called X-rays or gamma rays
      - The shortest wavelengths are gamma rays while the longest wave lengths are radio waves
      - Gamma rays are the most energetic waves while the least energetic waves are radio waves
        
        Therefore, the shorter the wavelength, the more energetic the wave is.
  - - - λ = 1500m
        
        Used in Broadcasting, Communication
    - - λ = 300m
        
        Used in Broadcasting, Communications
    - - λ = 25m
        
        Used in Broadcasting, Communiccation
    - - λ = 3m
        
        Used in Broadcasting, Communicatons
    - - λ = 30cm
        
        Used in TV transmissions
    - - λ = 3cm
        
        Used in Satellite transmission - Communication, Heating up food
        
        Hazard is it heats water in the body
        
        To prevent that, use a metal grid
    - - λ = 3mm
        
        Used in Communication in optical fibres, Remote Controllers, Heating, Night Vision
        
        Hazard is it is a heating effect - could burn skin
        
        To prevent that, use a reflective surface
    - - λ = 200-600nm
        
        Used in Seeing, Communicating, Photography, Optical Fibres
    - - λ = 100nm
        
        Used in Sterilising, Sun tanning, Fluorescent lamps
        
        Hazard is it causes damage to surface cells and blindness
        
        To prevent that, use sun cream (or cover up)
    - - λ = 5nm
        
        Used in Shadow pictures of bones, Observing Internal Structure
        
        Hazard is it causes cell damage - cancer
        
        To prevent that, use lead screens
    - - λ = <0,01nm
        
        Used in Sterilising Food and Medical Equipment
        
        Hazard is it causes cell damage - cancer
        
        To prevent that, thick lead screens or concrete
  - - - reflected, like light in a mirror
      - absorbed, like heat absorbed by a black surface
      - transmitted, or passed on, like light passing through glass
    - - When energy is absorbed by a surface it heats up.
        
        For example, microwaves are absorbed by water molecules and warm up
        
        Light waves pass through water
      - A dark painted metal surface absorbs radio and light waves
        
        X-Rays and gamma rays can pass straight through.
      - Wax can transmit microwaves, but absorbs light waves.
  - - - Brightly coloured animal advertise they are not good to eat
      - Semaphore flags used to be used to transmit messages
      - Smoke signals
      - Flashing Morse code messages with a torch.
    - - Telephones and computer links
    - - Therefore infra-red is used for telecommunication by optical fibres. Optical fibres are very flexible and allow the infra red signals to travel around corners
- - - - There is an incident and reflected ray
      - The angle of incidence and the angle of refraction are measured from the normal line
      - The normal line is a construction line that is at 90 degrees to the surface of the mirror
      - Law of reflection
        
        The angle of incidence = The angle of reflection
  - - - Some light is reflected
      - Some light is absorbed
      - Most of the light is transmitted (sent through the glass)
    - - The labels
        
        The angle of incidence is the angle the light ray makes with the normal
        
        The angle of refraction is the angle the refracted light ray makes with the normal inside the material
        
        The normal is a line drawn at right angles to the material's surface at the ray's point of entry
      - Observations
        
        The angle of incidence is greater than the angle of refraction. The ray therefore bends towards the normal
        
        When the ray emerges from the glass, it will bend away from the normal. The angle of refraction in this case is bigger than the angle of incidence
      - Refraction occurs because the speed of light is greater than the speed of light in glass
        
        The ratio of the sine of the angle of the incidence to the sine of the angle of refraction is a constant for a particular wavelength
        
        The ratio constant is called the relative refractive index 'n'
        
        The relative refractive index between two media where a light ray travels through one medium and is refracted through the other medium is given by
        
        n = sin(i)/sin(r)
        
        i = angle of incidence
        
        r = angle of refraction
  - - - 90degree deviation with a 90degree prism
        
        A light ray with an angle of incidence of 45 degrees will be reflected, and its angle of reflection will also be 45 degrees.
        
        Hence the light ray is deviated through an angle of 90 degrees
        
        For 90 degree deviation, a 90 degree isosceles glass prism is used. In this way the incident ray and the reflected rays are normal to the surfaces they enter and leave
        
        Total internal reflection occurs on the hypotenuse
      - 180degree deviation with a 90degree prism
        
        An isosceles prism is used to produce a 180 degree deviation, but reflection occurs on the equal, adjacent sides of the prism and not on the hypotense
        
        The light rays hit both sides at 45 degrees and reflect off each other to form a total 180 degree deviation
      - Optical Fibre
        
        Optical fibre typically consists of a core of high refractive index surrounded by cladding with a lower refractive index. Light is totally internally reflected down the fibre at the boundary of the two media
        
        The cladding increases the critical angle within the core fibre and also prevents adjacent fibres from touching each other.
        
        The fewer reflections, the less energy absorbed into the fibres and the shorter the time of transfer of information down the fibre since the light travels a shorter distance.
        
        The cladding allows light to escape into the surroundings but it's purpose is that only the light makes a small angle with the axis of the fibre is transmitted over long distances.
        
        The cladding increases the critical angles between the two materials. The benefits are
        
        only those rays that are close the axis of the fibre pass through
        
        There are fewer reflections and the distance travelled is smaller than the multiple reflection case and so there is less energy loss and the time of transmission is shorter
        
        The inner fibre is protected from damage
        
        The rays all travel roughly the same distance and so information fed in at one end arrives at the other only very slightly spread out in time
        
        Light suffers attenuation (reflection and absorption) in being transmitted alon gan optical fibre
        
        absorption and scattering by impurites
        
        irregularities at the boundary
        
        partial reflection at the ends
        
        bending of the fibre
      - Observations
        
        As the angle of incidence in the dense medium is increased, the angle of refraction increases towards 90 degrees. During this time, a weak reflected ray is observed
        
        When the angle of refraction is equal to 90 degrees, the angle of incidence is called the critical angle
        
        The critical angle (c) is the angle of incidence in a dense medium, such that the angle of refraction in the less dense medium is 90 degrees.
        
        Only when the angle of incidence in the medium exceeds the Critical Angle does all the light become reflected internally.
        
        The relationship between critical angle and refractive index
        
        sin(c) = 1/n
    - - A light ray with an angle of incidence greater than this will be totally internally reflected
- - - - Transverse Waves (Water Waves)
        
        The displacement of the medium is perpendicular to the direction of propagation of the wave.
        
        Ripple on a pond is an easily visualized transverse waves.
        
        Transverse waves cannot propagate in a gas or a liquid because there is no mechanism for driving motion perpendicular to the propagation of the wave.
      - Longitudinal Waves (Sound Waves)
        
        In longitudinal waves, the displacement of the medium is parallel to the propagation of the wave.
        
        A wave in a "slinky" is a good visualization.
        
        Soundwaves in air are longitudinal waves
        
        Parts of the wave
        
        Amplitude
        
        Height of a crest or depth of a trough
        
        Measured from the undisturbed position of what is carrying that wave
        
        Frequency (f)
        
        Number of complete waves generated per second
        
        Also can be defined as number of crests passing a point per second
        
        Unit - Hertz (Hz)
        
        Wavelength (λ)
        
        Distance between successive crests
        
        Unit - Metres (m)
        
        Period (T)
        
        time taken for one wave to pass a point
        
        Unit - seconds (s)
        
        Speed (v)
        
        distance moved in the direction of travel of the wave by a crest in 1 second
        
        Unit - metre/second (m/s)