(P6) WAVES

• Wavelength


  Distance between the same points on two consecutive waves
• Amplitude


  Distance from equilibrium line to the maximum displacement (crest or trough)
• Frequency


  The number of waves that pass a single point per second
• Period


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


  Velocity increases
• Wavelength increases


  Velocity increases
• Smaller period

  Higher frequency, greater velocity
• Transverse waves


  • Have peaks and troughs
  • Vibrations are at right angles to the direction of travel
• Longitudinal waves


  • Have compressions and rarefactions
  • Vibrations are in the same direction as the direction of travel
• For both transverse and longitudinal waves, the wave moves and not whatever it passes through
• Measuring velocity of sound in air

  1. Make a noise at ~50m from a solid wall, and 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, and record the time difference between a sound passing one to the other – then use speed = distance/time
• Measuring velocity of ripples on water surface

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

  2. Move a pencil along the paper at the same speed as a wavefront, and measure the time taken to draw this line – then use speed = distance/time
• 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 they appear matt and not reflective
  • The angle of incidence = angle of reflection
  • Light will reflect if the object is opaque and is not absorbed by the material

  
• Transmission


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


  • If the frequency of light matches the energy levels of the electrons
  • The light will be absorbed by the electrons and not reemitted
  • They will be absorbed, and then reemitted over time as heat
  • So that particular frequency has been absorbed
  • If a material appears green, only green light has been reflected, and the rest of the frequencies in visible light have been absorbed
• Sound waves in the ear

  1. Sound waves can travel through solids causing vibrations in the solid

  2. The outer ear collects the sound and channels it down the ear canal

  3. The sound waves hit the eardrum

  4. Compression forces the eardrum inward

  5. Rarefaction forces the eardrum outward, due to pressure

  6. The eardrum vibrates at the same frequency as the sound wave

  7. The small bones connected to this also vibrate at the same frequency (stirrup bone)

  8. Vibrations of the bones transmitted to the fluid in the inner ear

  9. Compression waves are thus transferred to the fluid (in the cochlea)

  10. The small hairs that line the cochlea move too

  11. Each hair is sensitive to different sound frequencies, so some move more than others for certain frequencies

  12. The hairs each come from a nerve cell

  13. When a certain frequency is received, the hair attuned to that specific frequency moves a lot, releasing an electrical impulse to the brain, which interprets this to a sound
• 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 are constant, so measuring the time between emission and detection can show distance from the source they are
  • A crack in a metal block will cause some waves to reflect earlier than the rest, so will show up
  • Scan of human foetus also use ultrasound for their non-invasive imaging
• Infrasound


  • It is a sound wave with a frequency lower than 20Hz – also known as seismic waves
  • P waves are longitudinal, and can pass through solids and liquids
  • S waves are transverse, only passing through solids (these move slower too)
  • On the opposite side of the Earth to an earthquake, only P waves are detected, suggesting the core of the Earth is liquid – hence no S waves can penetrate it
• Sonar


  • Pulse of ultrasound is sent below a ship, and the time taken for it to reflect and reach the ship can be used to calculate the depth
  • This is used to work out whether there is a shoal of fish below the ship
  • Or how far the seabed is below the ship
• Electromagnetic waves are transverse waves that do not need particles to move, and all have the same velocity (speed of light) in space
• As wavelength decreases


  Frequency must increase
• As frequency increases


  Energy of the wave increases
• Our retina can only detect visible light, a small part of the entire EM spectrum
• Refraction


  • If entering a denser material, it bends towards the normal
  • If entering a less dense material, it bends away from normal
• Substances will absorb, transmit, refract or reflect certain EM waves depending on wavelength
• When light enters a denser medium, it slows down, and shorter wavelengths slow down more than longer wavelengths
• Radio waves


  • Radio waves are produced by oscillations in electrical circuits
  • When radio waves are absorbed they create an alternating current, AC, at the same frequency as the radio waves
• Atoms and EM radiation


  1. When electrons change orbit (move closer or further from the nucleus)

  2. When electrons move to a higher orbit (further from the nucleus), the atom has absorbed EM radiation

  3. When the electrons falls to a lower orbit (closer to the nucleus), the atoms has emitted EM radiation

  4. If an electron gains enough energy, it can leave the atom to form an ion

  5. Gamma rays originate from changes in the nucleus of an atom
• Hazards of EM radiation

  • UV light, X-rays and gamma can have hazardous effects on human body tissue
  • The effects depend on the type of radiation and the size of the dose
  • UV – skin ages prematurely, increasing risk of skin cancer
  • X-ray and gamma are ionisation radiation that can cause the mutation of genes – causing cancer
• Lenses


  • If light passes through centre of lens, it does not change direction
  • Lenses are generally drawn as a dashed vertical line
  • Focal points are points either side of the lens which light can converge at
  • Convex lenses can have virtual or real images
  • Concave lenses can only have virtual images
• Concave lenses


  • They are thinner at centre than at edges
  • Spreads light outwards
  • Light appears to have come from the focal point
  • It is used to spread out light further, e.g. to correct short-sightedness
• Convex lenses


  • They are normally wider at centre
  • They focus light inwards
  • Horizontal rays focus onto focal point
  • Used for magnifying glasses, binoculars, and to correct long-sightedness
• Visible light spectrum

  • Blue has a shorter wavelength, and higher frequency, than red
  • Sunlight is a mix of all colours, and this mix appears white (i.e. white light is normal light)
• Specular reflection


  • Smooth surface gives a single reflection

  
• Diffuse reflection


  • Reflection off a rough surface causes scattering
• Colour filters


  • They work by absorbing every other colour and only letting certain wavelength (i.e. a certain colour) through
• Opaque colours


  • Wavelengths which are not reflected are absorbed
  • If all wavelengths reflect, it is white in colour
  • If all wavelengths are absorbed, it appears black
  • The wavelength which is absorbed = the colour which it appears
• Objects that transmit light are either transparent or translucent (scatter most light and only let some through)
• Black body radiation


  • All objects, regardless of temperature, emit and absorb infrared radiation
  • The hotter the body, the greater amount of radiation released per second and the greater amount of shorter-wavelength radiation released
  • A black body is an object that absorbs all the radiation it receives (does not transmit or reflect any) and therefore also emits all types of radiation
  • A body at constant temperature is still radiating/receiving radiation, but it is absorbing radiation at the same rate as it is emitting it
• The sun's energy is mostly absorbed by the earth's atmosphere, and some is reflected, which leads to the Earth's temperature