Waves

• The wavelength of a wave is the distance between two consecutive points on a wave that are in phase.
• A progressive wave carries energy from one place to another without transferring any material
• A wave is caused by something making particles or fields oscillate at a source.
• Parts of a wave

  
• Reflection is when the wave is bounced back after hitting a boundary
• Refraction is when the wave changes direction as it enters a different medium
• Em waves are transverse. They travel as vibrating magnetic and electric fields with vibrations perpendicular to the direction of energy transfer.
• A sound wave consists of alternate compressions and rarefactions of the medium its travelling through
• Polarisation can only happen for transverse waves
• Polarisation can only happen for transverse waves
• How does polarisation work

  1. ordinary light can be polarised using a polarising filter
  2. if you have 2 polarising filters at right angles no light will get through
  3. light becomes partially polarised when reflected from some surfaces
  4. if you reflected partially polarised light through a polarising filter at the correct angle you can block out unwanted glare eg polarised sunglasses.
• What is superposition


  When two or more waves cross the resultant displacement equals the vector sum of the individual displacements
• What is constructive interference?


  When two waves combine to create a larger amplitude.
• What is destructive interference?

  Cancellation of waves when they are out of phase.
• two points in a wave are in phase if they are both at the same point in the wave cycle.
• A complete cycle of a wave is 360 degrees or 2 radians
• Two points with a phase difference of zero or multiple of 360 are in phase
• Points with a phase difference of odd number multiples of 180 are exactly out of phase
• In order to get clear interference patterns, the two or more sources must be coherent and be in phase
• Two sources are coherent if they have the same wavelength and frequency and a fixed phase difference between them
• At any point an equal distance from two sources that are coherent and in phase you will get constructive interference
• But where the path difference is half a wavelength and one and a half etc the waves will arrive out of phase so therefore you will get destructive interference
• what is a stationary wave

  when the wave is in superposition of two progressive waves with the same frequency, moving in opposite directions.
• What is the first harmonic

  When the stationary wave is vibrating at the lowest possible frequency as it has one loop with a node at the end
• What is the second harmonic

  twice the frequency of the first harmonic with two loops and a node in the middle and one at each end
• What is diffraction

  The way waves spread out as they come through a narrow gap or round obstacles
• The amount of diffraction depends on the wavelength of the Wave compared with the size of the gap.
• To observe a diffraction pattern you need a monochromatic and coherent light source such as a laser.
• White light is a mixture of different colours with different wavelengths. So when white light is shone through a single narrow slit all of the wavelengths are diffracted by different amounts and you get a spectra of colours.
• the Central maximum in a single slit light diffraction pattern is the brightest part of the pattern as intensity is highest.
• The more photons per unit area hitting the central maximum per second than the other bright fringes.
• Increasing the slit width decreases the amount of diffraction as the central maximum is narrower
• Increasing the wavelength increases the amount of diffraction
• For grating with slits a distance apart, the angle between the incident beam and the nth order maximum is given by d sin（）=nlamda
• The larger the wavelength the more the pattern will spread out. The coarser the grating the less the pattern will spread out.