Waves

• waves transfer energy from one place to another without transferring matter (e.g. strumming strings on a guitar creates sound waves, the sound waves don't carry the air away from the guitar)
• Electromagnetic waves cause oscillations in electrical and magnetic field
• Mechanical waves cause oscillations of particles in a solid, liquid or gas and must have a medium to travel through
• Rest position - the undisturbed position of particles or fields when they aren't vibrating
• Displacement - distance a certain point in the medium has moved from its rest position
• Peak - highest point above the rest position
• Trough - lowest point below the rest position
• Amplitude - maximum length displacement of a point of a wave from its rest position
• Wavelength - distance between the same point on two adjacent waves (usually peak to peak or trough to trough)
• Frequency - number of complete waves passing a certain point per second (Hz)
• Period - time taken for a full cycle of the wave to pass a point ( T = 1/frequency )
• Wave Speed - the speed at which energy is being transferred (speed at which the wave is moving)
• Wave Speed (m/s) = frequency (Hz) x wavelength (m)
• Wave basics:

  A) peak

  B) amplitude

  C) wavelength

  D) wavelength

  E) wavelength

  F) trough

  G) rest position

  
• Speed of sound = 330m/s
• When sound is created, air particles vibrate and collide with each other, causing the vibrations to pass between the particles and eventually through to a person’s ear and vibrate their ear drum
• Light travels much faster through the air than sound
• You can measure the speed of air by having a person fire a starting pistol and raising their hand at the same time, then a distant observer 400m away can start a stopwatch when they see the person’s hand raise and stop it when they hear the sound
• Speed of sound (m/s) = distance (m) / time (s)
• Measure waves in a ripple tank by:

  • Setting up 5cm water depth.
  • Adjusting the vibrating bar to touch water surface.
  • Switching on lamp and power pack.
  • Recording underneath the ripple tank, capturing wave shadows on ruler and white paper.
  • Freezing recording to measure wavelength by dividing the distance between 10 waves by 10.
  • Slowing down recording, using a stopwatch to count waves passing a point in 10 seconds, and dividing by 10 for frequency.
  • Calculating wave speed with the formula: speed = frequency x wavelength.
• Measuring waves in ripple tank - electrical components near water - shock and damage to components - secure the electrical components before adding water taking care not to splash
• Measuring waves in a solid - (1) set up the equipment and turn on the signal generator and vibration transducer (2) adjust the frequency on the signal generator until you get stationary waves (3) measure the length of the half waves (loops) /  number of half waves (loops) x 2 to find the wavelength (4) find the wave speed by using wave speed = frequency x wavelength
• • Measuring waves in a solid - hazards - string snapping - damage to eyes - eye protection, safety screen
• • Longitudinal waves - oscillations are parallel to the direction of energy transfer, compressions are regions of high pressure due to particles being closer together, rarefactions - regions of low pressure due to particles being further apart, e.g. ultrasound, shock waves
• • Transverse waves - oscillations are perpendicular to the direction of energy transfer, e.g. electromagnetic waves, ripples
• Electromagnetic waves - transfer energy as radiation from a source to an absorber, can travel through a vacuum, their oscillations are changes in the electrical and magnetic fields perpendicular to the waves direction, transverse waves
• EM spectrum - radio waves - used to transmit radio and terrestrial TV signals, long wavelength radio waves diffract around the curved surface of the earth making it possible for radio signals to be received even if the receiver isn't in the line of site of the transmitter, shorter-wave radio signals can be reflected from the ionosphere so can also travel very long distances
• EM spectrum - Microwaves - used for heating food because most foods contain lots of water molecules which can easily absorb the energy of microwaves, the energy causes the temperature of the food to increase, used to communicate with satellites in spaces they can pass through the earth’s atmosphere without being reflected or refracted
• EM spectrum - infrared - emitted by electrical heaters and used to cook food in ovens as the energy of infrared is easily absorbed by the surface of objects, used for infrared cameras which detect the IR radiation and turns it into electrical signals which is displayed on a screen as a picture
• EM spectrum - visible light - used for communication and fibre optics, optical fibres are thin glass or plastic fibres which transmit pulses of light which carry informations, used to carry telephone and cable TV signals, short wavelength means they can carry lots of information
• EM spectrum - ultraviolet - used for energy efficient light bulbs, UV is created inside the bulb, energy is absorbed by the internal surface of the bulb and then converted into visible light (less energy than normal light bulbs as UV has a shorter wavelength and more energy than visible light), produced by the sun and can be used in tanning beds (with UV lamps) however this can increase the risk of cancer and premature skin ageing
• EM spectrum - X-rays - used for medical imaging, x-rays can be used to visualise broken bones as they can easily pass through flesh but not denser materials like bones or metals so it’s the amount of radiation that’s absorbed (or not) that gives you the x-ray image
• EM spectrum - gamma rays - used for medical imaging, can be used for medical tracers as as gamma emitting source can be injected into a patient and its progress can be followed
• EM spectrum - X-rays and gamma rays - can be used to treat cancers as in high doses they can kill all living cells so can be directed towards cancer cells, ionising radiation so can cause gene mutations, cell destruction and cancer so radiographers wear lead aprons or leave the room to keep their exposure to a minimum
• Electromagnetic spectrum:

  A) radiowaves

  B) microwaves

  C) infrared

  D) visible light

  E) ultraviolet

  F) x-rays

  G) gamma rays

  H) long wavelength, low frequency, low energy

  I) short wavelength, high frequency, high energy

  
• Lens - a shaped piece of transparent glass or plastic that refracts light
• Convex lens - When an object is placed 2F away from the lens the image is: real, inverted and diminished e.g. camera, human eye

  A) object

  B) focal length

  C) principal axis

  D) image

  
• Convex lens - When an object is placed between 2F and F the image is: magnified, introverted and real e.g. projectors

  A) object

  B) focal length

  C) principal axis

  D) image

  
• Convex lens - when an object is placed near than F to the lens the image produced will be: magnified, virtual and upright, the only time a convex lens produces a virtual image is when used in a magnifying glass

  A) image

  B) object

  C) focal length

  D) principal axis

  
• When a wave reaches a boundary between 2 different materials there things can happen, the wave can be: absorbed - energy is 'taken-in' by the material and the internal energy of the material will increase, transmitted - the waves travel through the new material, reflected - to throw back the wave without absorbing it