as level physics waves

• What is the definition of a progressive wave?


  A wave that transfers energy without transferring matter, and is made up of particles of a medium oscillating
• When are two points on a wave in phase?


  If they are at the same point in the wave cycle.

  They will have the same displacement and velocity.

  Their phase difference would be a multiple of 360.
• When are two points on a wave completely out of phase?


  When they're an odd integer of half cycles apart, where one half cycle is 180 degrees
• What is the definition of a transverse wave?

  Oscillation of particles that are perpendicular to their direction of energy transfer
• What are longitudinal waves?

  Oscillation of particles are parallel to the direction of energy transfer, and are made up of compressions and rarefactions (can't travel without a medium)
• Where does a polarised wave oscillate?

  In one plane
• What are the only type of waves that can be polarised and why?


  Transverse, the energy transfer is perpendicular to the direction of the oscillation of particles, meaning that the oscillations can be localised to a singular plane
• What does polarisation prove evidently for transverse waves?

  The perpendicular oscillation
• What are some examples of polarisation?

  Polaroid Sunglasses

  TV and radio signals
• What is superposition?


  The displacements of two waves are combined, and the resultant displacement is the vector sum of each waves displacement.
• What are the types of interference that can occur during superposition?

  Constructive and Destructive
• When does constructive interference occur?

  When the 2 waves have displacement in the same direction
• When does destructive interference occur?

  When one of the waves has a positive displacement and the other has a negative displacement (equal but opposite).
• How is a stationary wave made?


  Through the superposition of two progressive waves that are travelling in opposite directions

  They must have the same frequency, wavelength and amplitude
• Is energy transferred with a stationary wave?

  No
• What happens when two waves meet in phase?


  Constructive interference, antinodes form

  Anti-nodes are regions of maximum amplitude
• What happens when waves meet completely out of phase?


  Destructive interference, nodes are formed

  Nodes are regions of no displacement
• What is the first harmonic?

  The lowest frequency at which a stationary wave forms
• What is the distance between adjacent nodes/antinode for any harmonic?


  Half a wavelength
• What is the formula relating to harmonics?

  $f =$$\frac{1}{2L}\sqrt\frac{T}{\mu}$
• What do you do to the frequency to find an nth harmonic?


  Multiply the frequency by n
• What is path difference?


  The difference in distance travelled by two waves
• What makes a light source coherent?


  When the light has the same frequency, wavelength and a fixed phase difference
• Describe how to execute Young's double slit experiment


  Use a coherent light source and shine it through 2 slits around the size of the wavelength of light (to diffract the light)

  Each slit then acts as a coherent point source making a pattern of light and dark fringes (light interfering constructively and destructively)

  Light fringes are formed when the light interferes constructively and vice versa
• When are light fringes formed?

  When the path difference between waves is a whole number of wavelengths
• When are dark fringes formed?

  When the path difference between the waves is a whole number and a half wavelengths
• What is the associated formula for the double slit?

  $W =$$\frac{\lambda D}{s}$
• What happens when you diffract white light through a double slit?


  Wider maxima

  Less intense diffraction pattern

  Central white fringe

  Bright fringes are the rainbow (red being furthest away)
• What must you do if you don't have a coherent source for a double slit?


  Place a single slit before the double slit
• What acts as the evidence for the wave nature of light?


  Interference and Diffraction (wave properties)
• What is diffraction?


  The spreading out of waves as they pass through a gap
• When does the greatest diffraction occur?

  When the gap is the same size as the wavelength
• What does the diffraction of monochromatic light through a single slit appear like?


  Interference pattern (light and dark fringes)

  Bright central fringe (2x width of other fringes)

  Intensity decreasing from central fringe
• What does the diffraction pattern from white light through a single slit look like?


  Spectrum of colour appearing in fringes

  Central white maximum (2x width of other fringes)

  Violet closer to central max
• How do you vary the width of the central maximum?


  Increasing slit width (decreases amount of diffraction so central max becomes narrower)

  Increasing light wavelength (increases diffraction so central max becomes wider)
• What is a diffraction grating?


  A slide containing many equally spaced slits very close together, light diffracted through the grating is more sharp and bright due to more interfering rays
• What is the associated formula with diffraction gratings?


  $d sin\theta =$$n\lambda$
• How do you derive the diffraction grating formula?


  Consider first order maximum, path difference between two adjacent rays of light is one wavelength

  The angle between the normal to the grating and the light ray is theta

  Right angle triangle is formed with sides d and lambda

  Using trig, sin theta = lamda / d

  Other maxima occur when the path difference is n(lambda)

  Hence the diffraction grating equation
• What is a refractive index?


  A property of a material which measures how much light slows down when passing through it
• What is refraction?


  Wave changing direction upon entering a new medium

  The wave either bends towards or away from the normal depending on the new medium's refractive index