Topic 5: Waves

• What is amplitude?


  A wave's maximum displacement from the equilibrium position.
• What is frequency?


  The number of complete oscillations passing through a point per second.
• What is Period?


  The time taken for one full oscillation.
• What is speed in terms of waves?


  The distance travelled by the wave per unit time.
• What is wavelength?

  The length of one whole oscillation. (eg the distance between successive peaks/ troughs)
• Equation for speed of a wave

  Wave speed = frequency x wavelength
• What is a longitudinal wave?

  The oscillation of particles is parallel to the direction of energy transfer. Is made up of compressions and rarefactions and can't travel in a vacuum. An example is sound waves. They can be demonstrated by pushing a slinky horizontally.
• What are transverse waves?


  The oscillations of particles (or fields) is at right angles to the direction of energy transfer. All electromagnetic (EM) waves are transverse and travel at 3 x 10^8 ms^-1 in a vacuum. Can be demonstrated by shaking a slinky vertically or through the waves seen on a string, when it's attached to a signal generator.
• What is 'phase?'


  The position of a certain point on a wave cycle. Can be measured in radians, degrees or fractions of a cycle.
• What is phase difference?


  How much a particle/wave lags behind another particle/ wave. This can be measured in radians, degrees or fractions of a cycle.
• What is path difference?


  The difference in the distance travelled by two waves.
• What is superposition?


  Where the displacements of two waves are combined as they pass each other, the resultant displacement is the vector sum of each wave's displacement.
• What is coherence?

  A coherent light source has the same frequency and wavelength and a fixed phase difference.
• What is a wavefront?


  A wavefront is a surface which is used to represent the points of a wave which have the same phase.
• What are the two types of interference that can occur during superposition?


  Constructive and destructive
• What is constructive interference?


  This occurs when two waves are in phase so their displacements are added
• What is destructive interference?


  This occurs when the waves are completely out of phase and so their displacements are subtracted.
• What is 'in phase?'

  Two waves are in phase if they are both at the same point of the wave cycle, meaning they have the same frequency and wavelength and their phase difference is an integer multiple of 360. The waves don't need to have the same amplitude, only the same frequency and wavelength.
• What is 'completely out of phase?'

  Two waves are completely out of phase when they have the same frequency and wavelength and their phase difference is an odd integer multiple of 180.
• What is a stationary wave?

  Formed from the superposition of 2 progressive waves, travelling in opposite directions in the same place, with the same frequency, wavelength and amplitude. No energy is transferred by a stationary wave.
• What happens when a stationary wave meets in phase?

  Constructive interference occurs so antinodes are formed, which are regions of maximum displacement.
• What happens when a stationary wave meet completely out of phase?

  Destructive interference occurs and nodes are formed, which are regions of no displacement.
• What is intensity?


  The power (energy transferred per unit time) per unit area, and can be calculated using I=P/A. P = power, A = Area.
• What is the refractive index?

  A property of a materials which measures how much it slows down light passing through it. It is calculated by dividing the speed of light in a vacuum by the speed of light in that substance. A material with a higher refractive index can also be known as being more optically dense.
• When does refraction occur?


  Occurs when a wave enters a different medium, causing it to change direction, either towards or away from the normal depending on the material's refractive index.
• What is Snell's law?

  Used for calculations involving the refraction of light: n₁sinθ₁=n₂sinθ₂ -- where 'n' = refractive index of material and 'θ' is the angle of refraction of the ray in the material.
• What is total internal reflection?

  Can occur when the angle of incidence is greater than the critical angle and the incident refractive index is greater than the refractive index of the material at the boundary.
• What is total internal reflection?

  Can occur when the angle of incidence is greater than the critical angle and the incident refractive index (n₁) is greater than the refractive index of the material at the boundary (n₂)