P5

• Wavelength


  The distance between a point on one wave and the same point on the next. Typically measured in metres.
• Amplitude


  The distance from equilibrium line to the maximum displacement (crest or trough). Typically measured in metres.
• Frequency


  The number of waves that pass a single point per second. Typically measured in Hertz (Hz).
• Period


  The time taken for a whole wave to completely pass a single point. Typically measured in seconds.
• Velocity


  Frequency × wavelength
• Increasing frequency


  Increases velocity (directly proportional)
• Increasing wavelength


  Increases velocity (directly proportional)
• Period


  Inversely proportional to frequency, so the smaller the period, the higher the frequency and greater the velocity
• Transverse waves


  • Have peaks and troughs with the vibrations at right angles to the direction of travel. Light, or any electromagnetic wave are transverse waves.
• Longitudinal waves


  • Have compressions and rarefactions with the vibrations in the same direction as the direction of travel. Sound waves are longitudinal waves.
• Medium


  A substance the wave passes through, it could be air, water, glass etc.
• Density of a medium

  Refers to the optically density, not necessarily its physical dense.
• Frequency of a wave is always constant no matter of the medium.
• Passing into a denser medium

  The speed of the wave decreases, so wavelength decreases because the frequency is constant.
• Passing into a denser medium


  Speed decreases because it is travelling through a more dense medium, so it cannot travel as fast.
• The energy of the wave must be constant, because of conservation of energy. So this means frequency does not change, colour is dependent on frequency so colour stays the same.
• Reflection


  • Waves will reflect off a flat surface. The smoother the surface, the stronger the reflected wave is. Rough surfaces scatter the light in all directions, so appear matt and not reflective.
• The angle of incidence of a wave = angle of reflection.
• Transmission


  • Waves will pass through a transparent material. The more transparent, the more light will pass through the material. It could still refract, but the process of passing through the material and still emerging is transmission.
• Absorption


  • If the frequency of light matches the difference in energy levels of the electrons, the light will be absorbed by the electrons and not re-emitted except over time as heat.
• If a material appears a certain colour, only that coloured light has been reflected, and the rest of the frequencies in visible light have been absorbed.
• Ultrasound


  • When ultrasound reaches a boundary between two media, they are partially reflected back. The remainder of the waves continue and pass through. A receiver next to the emitter can record the reflected waves.
• The speed of the waves is constant, so measuring the time between emission and detection can show the distance from the source at which they were reflected.
• Ultrasound uses


  • Imaging under surfaces, e.g. A crack in a metal block will cause some waves to reflect earlier than the rest, so will show up. Foetus scan uses ultrasound as a non-invasive imaging method.
• Sonar


  • These waves undergo the same processes as ultrasound, but on a larger scale, e.g. Ultrasound sent from beneath the ship will bounce off the seabed and the time to receive the echo is used to calculate the depth to the seabed.
• How the ear works

  1. The outer ear collects the sound and channels it down the ear canal. As it travels through the canal, it acts as an air pressure wave. The sound waves hit the eardrum which is a tightly stretched membrane causing it to vibrate at the same frequency as the sound as the incoming pressure waves reach it.

  2. Compression forces the eardrum inward.

  3. Rarefaction forces the eardrum outward, due to pressure.

  4. The small bones (hammer, anvil and stirrup) connected to the drum then also vibrate at the same frequency. These small bones act as an amplifier of the sound waves and transfer the compression waves to the fluid in the cochlea.

  5. As the fluid moves, small hairs that line the cochlea move too. Each hair is sensitive to different sound frequencies, so some move more than others for certain frequencies.

  6. Each hair is attached to a nerve cell. When a specific frequency is received, the hair attuned to that frequency moves more, triggering an electrical impulse to the brain, which interprets this as the sound.
• Human hearing range


  20 - 20,000Hz
• The hairs in the cochlea attuned to the higher frequencies can easily die or get damaged, commonly caused by exposure to constant loud noise over a long period of time. It can also be due to the changes in the inner ear as you grow older. As a result, humans tend to lose the upper frequencies of their hearing range as they get older.
• We have evolved to hear this specific range of frequencies as it gives us the greatest survival advantage. We cannot hear ultrasound as we do not use sonar to hunt, instead, we have accurate vision.
• Ripple tanks


  • A ripple tank is a shallow glass tank containing a fluid (usually water) with a needle or paddle which oscillates, producing water waves at a chosen frequency. If light is shone through the tank, dark and light patches will appear underneath it as light passes through wave crests and troughs.
  • Troughs appear light.
  • Crests appear dark as there is a greater depth of water so the light is scattered the most.
• Calculating frequency in a ripple tank

  By counting the number of times a dark (maximum) passes through a point in a minute, then dividing this by 60 (seconds in a minute), the frequency (number of oscillations per second) can be calculated.
• Measuring wavelength in a ripple tank

  Wavelength can also be measured by using a strobe light at the same frequency as the waves, so the pattern of waves appears fixed on the screen. The distance between two maxima can be measured to calculate wavelength in the wave equation: λ = v/f
• Reflection in a ripple tank

  • Reflection can be shown in a ripple tank by using an obstruction in the tank.
• Refraction in a ripple tank

  • Refraction can be shown in a ripple tank by placing a thick glass sheet on part of the tank floor. The depth of water becomes shallower in that area. Since wave speed depends on depth, the ripples slow down in the shallow area. This is similar to how waves slow down when entering a denser medium.
• Particles travel perpendicular to the direction of the wave's travel. If a ping pong ball is placed in a ripple tank, it doesn't get carried by the wave, as the particles simply move up and down, not in the direction of the wave. This provides evidence that it is the wave travelling, not the water itself.
• Electromagnetic (EM) waves


  Transverse waves meaning they do not need particles to move to transfer energy. In space (a vacuum), all EM waves have the same velocity equal to the speed of light (3x10^8 m/s).
• Frequency-Wavelength Relationships


  Speed is constant for all EM waves. Therefore as wavelength decreases, frequency must increase and as frequency increases, the energy of the wave increases.
• Uses of each group of the electromagnetic spectrum

  • Radio - communications
  • Micro - cooking, as it heats the water or fat in foodstuffs
  • Infra-red - short range communication, remote controls
  • Visible - to illuminate things so we can see them
  • UV - sterilisation, as it kills bacteria
  • X ray - to see through soft tissue and look at skeleton
  • Gamma - used to kill cancer cells in radiotherapy
• UV, X-ray and Gamma are dangerous as they have a small wavelength, high frequency, and therefore high energy. This high energy means they can cause cells to mutate, potentially causing cancer.
• Radiotherapists, who constantly operate with gamma sources, try to maintain minimal exposure by leaving the room or wear lead aprons. Also pilots, flying at high altitudes where there is more UV, are more likely to suffer from cancer.