Physics: Waves, Sound & Light

WAVES
1. Longitudinal
  1. Particles vibrate parallel to the direction which the wave is travelling.
  2. Sound Waves, Ultrasound waves, Slinky spring waves & P-type earthquake waves.
2. Transverse
  1. Vibrations are at 90 degrees to the direction in which the wave is travelling.
  2. Water waves, slinky spring waves, waves on strings and ropes, electromagnetic waves.
3. Describing Waves
  1. Frequency = f : the number of complete waves passing a fixed point in a second. Measured in hertz.
  2. Wavelength: The distance between two consecutive crests or troughs. Measured in metres.
  3. Amplitude: the greatest displacement of the wave from its undisturbed position. Measured in metres.
  4. Wave speed: distance moved by a wave per second. Measured in m/s.
  5. WAVESPEED = FREQUENCY * WAVELENGTH
4. Wave fronts: Relfection and Refraction
  1. REFLECTION
    1. Angle of Incidence = Angle of Reflection
    2. When refection happens the wavelengths remain equal, the frequency remains equal and there is a continuity of incident and reflected waves at the barrier.
  2. Water waves are travelling from deep water to shallow water. Waves travel more slowly in shallow water than in deep water. Since the same number of waves number of waves leave the deep water as enter the shallow water every second, the frequencies in the waters must be the same - this means that the waves in the shallow water must have a shorter wavelength than those in deep water.
  3. The angle of incidence in deep water is always bigger than the angle of refraction in shallow water. The wavelengths & wave speeds in deep water are always bigger but the frequencies of waves in both waters are the same. continuity of both waves at the boundary.
  4. We can observe the behaviour of waves using a ripple tank or computer simulations.
Electromagnetic Waves
1. can travel in a vacuum and at exactly the same speed in a vacuum
2. are transverse waves
3. carry energy.
4. can be reflected and refracted.
5. Gamma Waves: O.O1nm
  1. Treatment of tumours, sterilising medical equipment and kill surface bacteria on foods.
  2. Can disrupt DNA in cells, and causes cancers.
6. X-Ray Waves: 0.1nm
  1. Diagnosis & therapy, body scanning in airports, scanning people for cancer.
  2. causes cancers & harmful to cells.
7. Ultraviolet Waves: 10nm
  1. Detect forgeries in banknotes & in water chillers to destroy bacteria which otherwise could build and cause disease and contamination.
  2. Skin cancers and damage to eyes.
8. Visible rays: 50nm
  1. Photosynthesis, vision & photography
9. Infrared: 0.01nm
  1. Toasters, ovens & grills. Night vision equipment and security systems for cars.
  2. Over exposure can cause damage to cells and sunburn or other skin burns.
10. Microwave: 3cm
  1. Mobile and satellite transmissions, microwave oven and navigation for planes and ships (GPS).
  2. Microwaves are absorbed by water molecules - this can harm our bodies as we are made up of water - to limit the risk, interlocks are fitted into the ovens to prevent production of microwaves when the door is left open.
11. Radio: 100m
  1. Communication: Television and FM Radios (use short wavelengths), and use with pilots, policemen, seafarers and military.
  2. Claims to cause cancers & leukaemia etc, not reliably proven.
SOUND
1. Sound at frequencies above 20kHz is called Ultrasound.
  1. Bell Jar Experiment explains why sound cannot travel through a vacuum as there are no particles and sound is created through changes in air pressure with the energy moving from particle to particle. The ringing bell is seen and heard from inside the bell jar. The pump is switched on and slowly all of the air is pumped out of the jar. and the sound is becoming fainter and fainter until it can hardly be heard at all.
  2. Infrasound is the range below human hearing.
2. Sound and Ultrasound can be made to reflect.
  1. Reflected Sound is called an Echo.
3. Ultrasound in medicine
  1. The scan of a foetus - a probe sends out ultrasound waves and detects reflections.
  2. Scanning soft tissues for diagnosis of cancer.
  3. Removing harmful tartar from teeth.
  4. Removing bladder stones by shattering them.
4. Ultrasound in Industry
  1. Used by geophysicists to detect vital materials like oil and minerals underground in rock.
  2. Scanning metal castings for faults & cracks.
  3. Fish location by sea-going trawlers.
  4. Oceanography to map the surface of the ocean floor.
5. Echo Principal
  1. Ultrasound is transmitted, and then reflected by boundaries, such as shoals of fish.
LIGHT
1. Reflection of Light
  1. Experiment 1: Reflection of Light pg87 CCEA book...
  2. Where is the imagine in the plane mirror? Learn experiment pg88 CCEA book..
    1. Properties of the imagine in a plane mirror
      1. Virtual
      2. The same size as the object
      3. Laterally inverted
      4. The same distance behind the mirror as the object is in front of the mirror
2. Travels at 300000000 m/s
3. Refraction of Light
  1. Light travels faster in air than in water, and faster in water than in glass
    1. Note: if the block has parallel sides, the angle of incidence is equal to the angle of emergence and a ray parallel to the normal does not bend as it enters the block.
    2. When light speeds up, it bends away from the normal and when light slows down it bends towards the normal.
    3. As the light enters the glass at point A it slows down, so it bends towards the normal
      1. As the light passes from glass to water at point B it speeds up so it bends away from the normal.
        As the light passes from water into air at point C it speeds up further, so it bends still more away from the normal.
    4. Refraction of light experiment in workbook.
  2. Analogy between water waves and light.
4. Dispersion of Light
  1. All colours of light travel at the same speed in air, but different speeds in glass, therefore the colours bend by different amounts when they pass from air to glass. When light passes through a prism, the effect is called dispersion and it results in a spectrum. Red light is bent the least because it travels the fastest in glass and violet light bends the most because it travels the slowest in glass.
5. Total Internal Reflection
  1. within optical fibres, light strikes the core-cladding boundary at an angle bigger than the critical angle and be totally internally reflected at the surface of the glass core.
    1. However every optical fibre has its imperfections at its reflecting surface and this means that the signal must be boosted very km or so in communication links.
  2. Optical fibres enable long-distance communications because light reflects and passes through the fibre with very little attenuation compared to a normal metal cable.
    1. If the optical fibre is bent too tightly, the angle of incidence at the core cladding boundary may become less than the critical angle and light will be lost by refraction into the cladding.
    2. Endoscopes use total internal reflection to look inside a patients body without cutting a huge hole. Endoscopes consist of bundles of fibres that allow light to travel into the body and then image information to pass out of the body.
    3. Prism binoculars.
    4. Prism periscopes.

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Physics: Waves, Sound & Light

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	Created by orla connolly about 8 years ago