5 Waves and particle nature of light

• Superposition

  Vector sum of displacements of two waves at a point where they meet
• How a standing wave is formed

  • Waves are reflected
  • When they meet they superpose
  • Where they meet in phase, constructive interference occurs and it produces antinodes
• What is the difference between longitudinal and transverse waves

  Longitidunal is when waves whose oscillations are parallel to the direction of energy transfer. consists of compressions and rarefractions (eg. sound waves)

  Transverse waves are waves whose oscillations are perpendicular to the direction of energy transfer (eg. EM waves)
• When does constructive interference occur

  When two waves are in phase

  form a max amplitude known as a antinode

  Path difference = nλ
• Phase difference

  the differences in starting between waves. Measured in 2 π, π ,π /2
• Path difference

  also differences in starting but measured in λ(m)
• Wave front

  surface of a wave where the points all have the same phase
• Diffraction

  spreading out of waves when they pass through a gap
• Diffraction depends on gap width and wavelength of wave

  • gap size is wider than wavelength diffraction is unnoticed
  • gap size is similar to wavelength, diffraction is greatest
  • gap size is smaller than the wavelength, most waves are reflected
• Huygens principle

  He says

  Every point on a wavefront is a point source to secondary wavelets, which spread out to form the next wavefront

  these further waves superpose
• Waves are in phase when they are coherent ( same frequency and wavelength ), amplitude don't need to be the same

  path difference is an integer multiple of 360 degrees = 2π, 4π
• Waves are in antiphase when they are coherent ( same frequency and same wavelength )

  But their path difference is an odd integer = π/2 , 3π/2

  (n + 1/2)λ
• Stationary waves

  Superposition of two waves of the same frequency and amplitude travelling in opposite directions
• Superposition

  Two waves meet

  The displacement is the sum of the individual displacements from the individual waves
• De brogile

  λ = h/p
• Real image

  Light rays pass through the image
• Focal length ( f )

  1/f = 1/u + 1/v

  distance from centre of lens to principle focus
• Magnification

  m = v/u or Image height (measured) / Object height (given)
• Diverging lens

  Curved inwards, they cause parallel light rays to move apart/diverge

  
• Converging lens

  Curved outwards, they cause parallel light rays to move closer together/converge at a point

  
• Angle of deviation

  Angle of refraction - Angle of incidence
• " If distance from lens to screen is doubled, the brightness of the image is halved "

  Statement is incorrect as the power is spread over 4 times the area

  so brightness falls to one quarter

  I = P/A 2(2pi r^2)
• Power of lens

  1/f

  Power is positive in a converging lens and negative in diverging lens
• Thin lens

  P = P1 + P2 + P3

  A thin lens is with a thickness which allows rays of light to refract but not experience dispersions or aberrations

  they act as a single lens
• Virtual image

  cannot be projected on a screen as light rays do not form an image
• Explain what the student observes as he gradually rotates the filter to 180° then to 270°

  • At 180° the screen is normal
  • When oscillations are parallel to the filter all the light is transmitted
  • At 270° the screen is dark
  • When oscillations are perpindicular to the filter all the light is absorbed
  • So there is a change in amount of light absorbed or transmitted as the filter is rotated
  • Light from the screen is polarised
• State what is meant by polarised light

  Oscillations are in a single plane

  Which includes the direction of propagation
• Explain how the plane of polarisation of emitted light can be demonstrated using a polarising filter

  • Rotate the filter
  • Screen is brightest when plane of polarised light is parallel to the plane of polarisation of the filter
  • As polarised light from the screen is transmitted by the filter
• Nλ = dsinθ

  d = 1/ lines per mm

  N = 1,2,3,4

  θ use Pythagoras
• Explain why a series of bright dots is seen on the screen.

  • Waves superpose
  • bright dots mean the waves are in phase
  • Constructive interference occurs
• state what is meant by polarised light

  The oscillations are in a single plane, which includes the direction of propagation
• Unpolarised light 


  Oscillations are in more than one plane, which include the direction energy of transfer
• Refraction


  Change in direction of wave as it travels through a medium which leads to change in wave speed and wave length

  Frequency is constant