waves

• waves may either be transverse or longitudinal
• waves can be described as oscillations about a rest position
• transverse waves:

  • oscillations happen at right angles to the direction of energy transfer
  • have peaks and troughs
• longitudinal waves:

  • oscillations are parallel to the direction of energy transfer
  • have compressions are rarefractions
• wave speed = frequency X wavelength
• time period = 1 / frequency
• waves transfer energy from one place to another without transferring matter.
• in a wave, energy is transferred through particles that oscillate. Because the particles oscillate, it is evidence that it is the wave that and not the water or air that travels, because the particles just vibrate back and forth about a fixed position. This means that there isn't a movement of matter in the direction of the wave, just a movement of energy.
• the amplitude of a wave is the maximum displacement of a point on a wave from its undisturbed position.
• the wavelength of a wave is the distance from a point on one wave to the equivalent point on the adjacent wave
• the wave speed is the speed at which energy is transferred (or the wave moves) through a medium
• examples of transverse waves: ripples on the surface of water, S-waves, electromagnetic waves
• examples of longitudinal waves: sound waves (including ultrasound waves), P-waves,
• method to measure speed of sound in air (1) --

  1. measure a large distance of 300-400m using a trundle wheel
  2. have a person stand on either end of the distance
  3. start a stopwatch when you see the person clash cymbals together
  4. stop the stopwatch when you hear the clash
  5. speed = distance / time
• method to measure speed of sound in air (echoes) --

  1. stand 50 metres away from a wall, facing it
  2. use bricks and bang them together to make a sound. Drop the bricks after
  3. when you hear the echo, clap, and repeat the process
  4. have someone else start a stopwatch, and stop after hearing 20 claps
  5. the process should be repeated 3 times; an average time calculated
  6. distance travelled by the sound between each clap and echo will be 2X50 (there and back)
  7. the total distance travelled by the sound will be 20 (number of claps) X 2 X 50 (distance travelled each clap)
  8. speed= total distance/time
• measuring wave speed in water --

  1. chose somewhere with calm, flat water such as a pond or swimming pool
  2. two people should stand 5metres apart, using a tape measure to measure the distance
  3. one person counts to 3 and then disturbs the water surface using their hand to create a ripple
  4. the second person starts a stopwatch after 3 and measures the time taken for the ripple to reach them
  5. steps 3 and 4 should be repeated 10 times, leaving adequate time in between (1-2 minutes) for the water to become calm again
  6. calculate an average time
  7. speed = distance (between them) / average time
• • sound waves are longitudinal waves
  • and they transfer energy by molecules vibrating and knocking into neighbouring molecules
  • so the more molecules that are present (the denser the medium), the faster energy can be transferred
  • therefore, sound waves travel fastest in solids
  • and slowest in gases
• • the frequency of a wave stays the same as it moves through different mediums
  • but the wavelength changes
  • and therefore the wave speed changes (wave speed = wavelength x frequency)
  • and this change in velocity (wave speed) can lead to a change in the wave direction
  • when this happens, it is known as refraction
• changes to velocity, frequency and wavelength are inter-related through the equation:

  wave speed (velocity) = wavelength x frequency
• refraction of sound waves -- more to less dense:

  • wave speed decreases (less particles to oscillate)
  • frequency stays the same
  • wave length decreases
• refraction of sound waves -- less to more dense:

  • wave speed increases
  • frequency stays the same
  • wave length increases
• the speed of sound in air can also be affected by temperature --

  • on warm days:
  • air particles have more energy
  • so oscillate faster (knock into neighbouring particles more quickly)
  • so energy is transferred quicker, and sound wave is carried faster
  • increasing the speed of sound
  • on cold days:
  • air particles have less energy
  • so they oscillate at a slower pace (knock into neighbouring particles slower)
  • so energy is transferred at a slower pace, and the sound wave is carried slower
  • decreasing the speed of sound
• depending on the density of the mediums either side of a boundary, a wave may be:

  • absorbed
  • transmitted
  • reflected
• reflection occurs when a wave hits a boundary between two media and does not pass through, but instead stays in the original medium.

  • the law of reflection states the angle of incidence = the angle of reflection
• when a wave is reflected, some of it may also be absorbed or transmitted
• rough surfaces are the least reflective. this is because the light scatters in all directions, so they appear matt and unreflective.
• smooth surfaces are the most reflective, because the smoother the surface, the stronger the reflected wave is
• opaque surfaces will reflect light which is not absorbed by the material, as the electrons will absorb the light energy and then reemit it as a reflected wave
• transmission occurs when a wave passes through a substance
• transmission:

  for light waves - the more transparent the material, the more light will pass through
• sound waves can travel through solids, causing vibrations in the solid
• waves do not change frequency at a boundary because to change the frequency waves would have to be created or destroyed at the boundary which is not possible
• the range of normal human hearing is 20Hz to 20kHz (20,000Hz)
• ultrasound is sound waves with a frequency above the upper limit of human hearing of 20kHz (20,000Hz)
• ultrasound waves are partially reflected at a boundary between two different media
• ultrasound imaging:

  • an ultrasound detector is made up of a transducer that produces and detects ultrasound waves
  • ultrasound waves are partially reflected at the boundary between different tissue
  • and this reflected wave is hit the transducer
  • the time taken for the wave to be detected after it's left the transducer is measured
  • and the depth of the boundary can be determined using the speed of sound in the material and time taken (distance = (speed x time) / 2)
  • by taking a series of ultrasound measurements, the distance can be used to form an image
• echo sounding:

  1. echo sounding uses ultrasound to detect objects underwater
  2. an ultrasound wave is emitted from a boat, and partially reflected off the sea bed
  3. the time taken for the wave to be detected after emission can be used to calculate the distance
  4. distance = (speed x time) / 2
• seismic waves --

  • P-waves:
  • longitudinal waves
  • and so can pass through solids and liquids
  • S-waves:
  • transverse waves
  • so can only pass through solids
  • when there is an earthquake or seismic activity in which S-waves and P-waves are emitted, P-waves can be detected on the opposite side of the Earth but S-waves cannot be detected
  • therefore meaning that the Earth's core must be liquid, because S-waves cannot travel through liquid
  • the waves are also refracted as they pass through the Earth, meaning there are different densities and media in the Earth, or different layers
• electromagnetic waves are transverse waves that transfer energy from a source of the waves to an absorber.
• electromagnetic waves form a continuous spectrum and all types of electromagnetic waves travel at the same velocity (speed) through a vacuum or air. The waves form the electromagnetic spectrum and are grouped in terms of their wavelength and frequency.