Waves

• when waves travel through a medium, the particles of the medium oscillate and transfer energy between each other
• the amplitude of a wave = the maximum displacement of a point on the wave from its undisturbed position

  the wavelength = the distance between the same point of two adjacent waves
• in transverse waves, the oscillations are perpendicular to the direction of energy transfer

  most waves are transverse, e.g. all electromagnetic waves, ripples and waves in water, a wave on a string
• in longitudinal waves, the oscillations are parallel to the direction of energy transfer

  e.g. sound waves, shock waves (like seismic waves)
• by attaching a signal generator to a speaker, you can generate sounds with a specific frequency

  you can use two microphones and an oscilloscope to find the wavelength of the sound waves generated (the wavelength is the distance between the microphones)
• measuring the speed of water ripples using a lamp:

  -using a signal generator attached to the dipper of a ripple tank

  -the lamp can be used to see wave crests on a screen below the tank

  -the distance between each shadow line is equal to one wavelength (measure the distance between shadow lines that are 10 wavelengths apart, and divide this distance by 10 to find the average wavelength)
• working the speed of wavelengths on strings:

  -equipment: signal generator, vibration transducer, string attached to vibration transducer connected to masses on a pulley

  -turn on the signal generator and vibration transducer, this will cause the string to vibrate

  -adjust the frequency of the signal generator until there's a clear wave on the string (the frequency will depend on the length of the string between the pulley and transducer, and the masses)

  -measure the lengths of four/ five half-waves in one go, then divide to get the mean half-wavelength. Then double this to get a full one
• angle of incidence = angle of reflection

  
  

  angle of incidence = angle between the incoming wave and the normal

  angle of reflection = angle between the reflected wave and the normal

  
  

  the normal is an imaginary line that's perpendicular to the point of incidence (where the wave hits the boundary)
• electromagnetic wave spectrum:

  -R adiowaves

  -M icrowaves

  -I nfrared

  -V isible

  -U ltra violet

  -X -rays

  -G amma
• -When a wave crosses a boundary between materials at an angle, it changes direction (is refracted)

  -how much its refracted depends on how much the the wave speeds up or slows down, which usually depends on the density of the two materials

  -the wavelength changes, but the frequency stays the same

  -less dense to denser --> the wave hits a different medium at an angle, so changes direction

  -less dense to denser --> hits the medium straight, wave fronts are closer together (shorter wavelength)
• Radio waves: Used for radio and television broadcasting, mobile phone networks and satellite communications
• Radio waves:

  -are made up of oscillating electric and magnetic fields

  -alternating currents

  -can be produced using an alternating current in an electrical circuit (the object in which charges, electrons, oscillate to create the radio waves called a transmitter)

  -when reach a receiver, are absorbed --> the energy causes the electrons to oscillate and, if the receiver is part of a complete electrical circuit, generates an alternating current
• Radio waves:

  -are EM radiation with wavelengths longer than 10cm

  -long-wave radio (wavelengths of 1-10 KM) --> bend around the curved surface of the Earth: this makes it possible for radio signals to be received even if the receiver isn't in line of the sight of the transmitter

  -short-wave radio signals --> can also be received at long distances as they are reflected from the ionosphere; uses include bluetooth
• ionosphere = an electrically charged layer in the Earth's upper atmosphere
• Microwaves --> for satellites (can pass easily through the Earth's atmosphere); for microwave ovens (microwaves are absorbed by water molecules in food, absorption of energy)
• Infrared radiation --> given out by all hot objects: uses include infrared cameras and electrical heaters, and cooking food
• Fibre optic cables --> are thin glass or plastic fibres that can carry data (e.g. from telephones or computers) over long distances as pulses of visible light

  they work because of reflection, as the light rays are bounced back and forth until they reach the end of the fibre
• x-rays: pass through flesh, absorbed by bones --> can also be used, along with gamma radiation, to treat cancer (radiotherapy) as they can be carefully directed towards cancer cells to kill them

  gamma radiation can also be used as a medical tracer: a gamma-emitting source is injected into the patient and its progress is followed around the body
• radiation dose (measured in sieverts) is a measure of the risk of harm from the body being exposed to radiation

  1000 mSv = 1 Sv
• black bodies:

  -objects that absorb all the radiation that hits it (no radiation is reflected or transmitted)

  -are the best possible amitters
• sound waves are caused by vibrating objects- these vibrations are passed through the surrounding as a series of compressions and rarefactions

  
  

  therefore sound cannot travel through space, as it's mostly a vacuum (therefore there are no particles to move or vibrate)
• ultra waves --> when it passes from one medium into another, some of the wave is reflected off the boundary meanwhile some is transmitted/ refracted (this is a partial reflection)

  this means you can point a pulse of ultrasound and wherever there are boundaries, some of the ultrasound is reflected back

  the time it takes for the reflections to reach a detector can be used to measure how far the boundary is
• p-waves = longitudinal waves, travel through solids and liquids, travel faster than S-waves