Waves

• • All waves have a:
  • frequency - the number of waves passing a fixed point per second, measured in hertz (Hz)
  • amplitude - the maximum displacement that any particle achieves from its undisturbed position in metres (m)
  • wavelength - the distance from one point on a wave to the equivalent point on the next wave in metres (m)
  • period - the time taken for one complete oscillation in seconds (s).
• Electromagnetic Waves:

  Refraction is due to the difference in the wave speed in the different media.

  • When a light wave enters, at an angle, a medium in which it travels slower:
  • the first part of the light wave to enter the medium slows down
  • the rest of the wave continues at the higher speed
  • this causes the wave to change direction, towards the normal.
• Ray Diagrams:

  • When waves reach a boundary between one medium (material) and another, they can be refracted.
  • When constructing a ray diagram:
  • Rays must be drawn with a ruler.
  • Each straight section of ray should have a single arrow drawn on it to indicate the direction of movement.
  • Where the ray meets the boundary, a 'normal' should be drawn at right-angles to the boundary.
  • All relevant angles should be labelled.


  Ray Diagram:

  
• Required Prac: Identify the suitability of apparatus to measure the frequency, wavelength and speed of waves in a ripple tank.
• RP: Sample Method

  1. Time how long it takes one wave to travel the length of the tank. Use this to calculate wave speed using speed = distance/time
  2. To find the frequency, count the number of waves passing a fixed point in a second.
  3. Estimate the wavelength by using a ruler to measure the peak-to-peak distance as the waves travel.Use a stroboscope to make the same measurements and compare the results.
• RP: Considerations, Mistakes and Errors

  • Using a stroboscope can significantly improve the accuracy of measurements.
  • By projecting a shadow of the waves onto a screen below the stroboscope, flash speed can be adjusted to make the waves appear stationary. This makes wavelength measurements much more accurate.
  • For high frequencies that are difficult to count, this can be used with the wave speed measurement to calculate the frequency using wave speed = freq x wavelength
• RP: Hazards and Risks

  • When using a stroboscope there is a risk to people with photo-sensitive epilepsy. It is important to check that there are no at risk people involved in the experiment or in the area.
• RP: variables

  • The key control variable is water depth. It important to ensure that the depth of the water is kept constant across the tank as, for a given frequency, the depth will affect the speed and wavelength.