Waves

• Longitudinal waves

  • Sound waves
  • Have compressions and rarefactions
  • Vibrations are in the same direction as the direction of travel
• Types of Waves

  • Transverse waves
  • Longitudinal waves
• Increase frequency

  Velocity increases
• Wavelength increases


  Velocity increases
• Waves can be reflected, absorbed, or transmitted at the boundary between two different materials
• Sound Waves

  • Can travel through solids causing vibrations
  • Outer ear collects sound and channels it down the ear canal
  • As it travels down, it is still a pressure air wave
  • Sound waves hit the eardrum
  • Eardrum is a tightly stretched membrane which vibrates as the incoming pressure waves reach it
  • Compression forces the eardrum inward, rarefaction forces it outward
  • Eardrum vibrates
• Amplitude


  Distance from equilibrium line to the maximum displacement (crest or trough)
• Reflection of waves

  1. Waves will reflect off a flat surface

  2. The smoother the surface, the stronger the reflected wave is

  3. Rough surfaces scatter light in all directions, so they appear matt and not reflective

  4. The angle of incidence = angle of reflection

  5. Light will reflect if the object is opaque and not absorbed by the material
• Transverse waves

  • Light, or any electromagnetic wave
  • Have peaks and troughs
  • Vibrations are at right angles to the direction of travel
• Transmission of waves

  1. Waves will pass through a transparent material

  2. The more transparent, the more light will pass through the material

  3. It can still refract, but the process of passing through the material and still emerging is transmission
• Measuring velocity of ripples on water surface

  1. Use a stroboscope with the same frequency as the water waves, measure distance between 'fixed' ripples and use v = fλ

  2. Move a pencil along the paper at the same speed as a wavefront, measure the time taken to draw this line, then use speed = distance/time
• Period


  The time taken for a whole wave to completely pass a single point
• Frequency


  The number of waves that pass a single point per second
• For both types of waves

  The wave moves and not whatever it passes through
• Wavelength


  Distance between the same points on two consecutive waves
• Measuring velocity of sound in air

  1. Make a noise at ~50m from a solid wall, record time for the echo to be heard, then use speed = distance/time

  2. Have two microphones connected to a datalogger at a large distance apart, record the time difference between a sound passing one to the other, then use speed = distance/time
• Evolution has led humans to hear a specific range of frequencies for survival advantage
• Infrasound is a sound wave with a frequency lower than 20Hz, used to explore the Earth's core
• All electromagnetic waves have the same velocity in space (speed of light) and can transfer energy from a source to an absorber
• Our retina can only detect visible light, a small part of the entire EM spectrum
• In the cochlea, hairs attuned to higher frequencies can die or get damaged due to various reasons
• Ultrasound waves are partially reflected back when reaching a boundary between two media
• Humans cannot hear below 20Hz or above 20kHz
• As frequency increases

  Energy of the wave increases
• Electromagnetic waves are transverse waves that do not need particles to move
• As speed is constant for all EM waves

  As wavelength decreases, frequency must increase
• How sound travels through the ear

  1. Sound enters the ear canal as a pressure air wave

  2. Sound waves hit the eardrum, causing it to vibrate

  3. Vibrations of the bones transmit to the fluid in the inner ear

  4. Compression waves are transferred to the fluid in the cochlea

  5. Hairs in the cochlea move according to different sound frequencies

  6. Nerve cells release electrical impulses to the brain for interpretation as sound
• Sonar uses ultrasound to calculate depth below a ship and detect shoals of fish or the seabed
• Ultrasound usage

  1. Ultrasound waves can be used for imaging under surfaces

  2. Ultrasound waves can show cracks in materials

  3. Ultrasound is used in non-invasive imaging of human foetuses
• Refraction

  1. If entering a denser material, it bends towards the normal

  2. If entering a less dense material, it bends away from normal

  3. Substances will absorb, transmit, refract or reflect certain EM waves depending on wavelength
• Radiation dose determines how much exposure leads to harm for a person
• If an electron gains enough energy, it can leave the atom to form an ion
• As wavelength decreases, frequency must increase


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• Lenses are generally drawn as a dashed vertical line
• Focal points are points either side of the lens where light can converge
• Dispersion occurs of white light into a prism because different wavelengths refract a different amount, creating a rainbow effect
• Radio waves are produced by oscillations in electrical circuits
• Gamma rays originate from changes in the nucleus of an atom
• Convex lenses can have virtual or real images
• Uses of EM waves include TV, radio, satellite communication, cooking food, infrared cameras, fibre optics, sun tanning, medical imaging, and treatment